# Solve Sudoku (Without even thinking!)

This instructable provides step by step instructions to complete a Sudoku puzzle by simple process of elimination.

One of the things I that drove me crazy about Sudoku is how difficult it is to return to a puzzle if you get interupted. This system allows you to walk away from a puzzle at any time and return exactly where you left off.
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## Step 1: What you need to begin

You need:

1. A sudoku puzzle. I enlarged this one (which was the puzzle that appeared on Sudoku.com on July 8, 2006.) to make photography easier. I find this system works better on larger puzzles.

2. A pencil, Not a pen. A pencil.

3. An eraser.

## Step 2: Fill in the "Missing Grid"

You see, a Sudoku puzzle is missing a grid. Once you put in this grid, it is sooo much easier.

Enter the numbers 1 through 9 in a "tic-tac-toe" pattern in every blank box.

## Step 3: Erase "Across"

For each number that is preprinted in the puzzle, you will start erasing that number from the appropriate grids you just wrote in.

So the first number (in the upper right corner) is a "6". Erase every other "6" in that row. Once you have completed the erasing, draw a line across the top of the number to indicate that you have cleared out that line.

## Step 4: Erase Down

Now, for that same number "6", erase the corresponding "6"'s in every box in the same column. Once you have completed erasing, draw a line down the edge of the number to indicate that you have completed this step.

## Step 5: Erase "All Around"

Ok, last step for the first number "6" Erase all the "6"s in the same quadrant. Then draw a circle around the number to indicate you have completed this step.

Now you have eliminated all the sixes from all the other blocks. That indicates that a Six canot appear in any of those blocks.

Keep going, repeat those same three steps for all the other pre-printed numbers in the puzzle.
wlaraby1 month ago
Sudoku is simply a logic puzzle. If it's here, it's not there. May not be the fastest way, but I average 2.5-3.5 minutes on an easy puzzle. Just start with 1, find the first 1 and look across, look down, if there are two 1's in that column or row, and only one spot to put a 1 in the remaining block do so. When no easy option presents itself move on to the 2's, and repeat. Another thing to look for is say the top left block has a 1 in the top left, the top middle block is filled except the middle row across but no 1, and the top right block middle row is empty and the bottom right square is empty.
You know the middle block has to be the middle row somewhere, but not where. That said you can fill in the square for the right hand block because you know the middle row is taken.
Last for the comment, look for rows, columns, or blocks with only 2-3 empty spaces when your stumped. You may see that it needs a 1, and a 5, but one of the spaces in a line already use the 5. So you can put the one in that spot and the 5 in the other.
Maybe I'll actually write up my own how too. Ya think?
MattM81 month ago

If anyone is lost or stuck, you can try my simple sudoku solver!

http://www.simplesudokusolver.com/

Should be really fast and easy to use. Let me know if you have any feedback!

Mercy MeS2 months ago

Alright its still a god technique but its not fast enough. I need a fast one

Mercy MeS2 months ago

Alright its still a god technique but its not fast enough. I need a fast one

Mercy MeS2 months ago

Wow! I was struggling trying to find a technique to help me with Sudoku and this was it. It was simple and straight to the point and it does work with the hard ones. The only thing wrong is that you do have to think...............TO PICKUP YOUR PENCIL AND ERASE! That is all I thought about. Try this it works

sudokusolvertip2 months ago

Nice one! You are implicit using some of the solving techniques mentioned here:

http://sudokusolvertips.com/category/sudoku-resolving-strategies/

lathishR4 months ago

aaawwwwwwesome

vincent75207 months ago

without thinking is preposterous as the "solution" you give is precisely what we all do when we solve à sudoku ... Exception we do it in our brain and do not write it down.

7 months ago

sorry "except" not "exception"

Difficult to write english without an french auto correct !!!! ;)

chocolateface1 year ago
What's the point in solving a sudoku without thinking? There's no pride or "retribution".
R6LPW1 year ago
Using computer programmes to solve Sodoku seems to defeat the object of trying to solve a puzzle or problem to exercise the brain!
Presumably 'broper's somewhat exotic spelling is not for real!
(who is Ible?)
nafango226 years ago
this doesnt work with very hard sudoku puzzles. the really tricky ones will make you randomly pick a number for a square and check to make sure it works in that square as you fill more in.
5 years ago
I have done many "genius level" Sudokus without guessing. There is no need to guess. If you are interested, I'll put up the information on a wordpress blog that works for me.
1 year ago
Snowbunny7 years ago
I picked up my first puzzle and applied this technique. I failed then copied the puzzle again and took my time. Unfortunately the puzzle was labeled "gold" or "Hard" and I still have a few squares left with a lot needing the same numbers. Is there some sort of "next step" or should I just "hit and miss" and take the "keep trying approach"? Any suggestions?
5 years ago
No need to guess. If you are interested, I can provide additional clues.
1 year ago
flarr1 year ago
whats the matter with some of you,this is the easyiest systemi have ever seen I just solved the outlaw level3 times with out a hitch,simple just fallw instructions thank you

love this system
sankalpsawant1 year ago
what to do when 1 to 9 numbers already exist in column & row of particular block....
sankalpsawant1 year ago

man pscovio
deau_oow2 years ago
Sudoku mean a mathematics, brainstorming and calculation. you are completely wrong if "Don't even to think" for sudoku. This is kind of joke of the day for me. by using your above 10 step technique, only simple sudoku's could be solved not even medium, forgot about hard, tough and evils. not even 2% of the puzzle will be solved using these approach.
iyyanar45652 years ago
but i need quick method
2 years ago
There is no quick method. You're supposed to relax and think.
iloveswing952 years ago
If you guys have an iphone or ipod I thought Pennypop's sudoku app had a really nice learning curve. I picked it up pretty fast and I'm not really that smart :P. it's free too and looks way better than the other apps.
rjvkshl2 years ago
I find SUDOKU to be a very refreshing and mind-invigorating game. If I solve a puzzle with my morning cuppa tea, it helps me to clear my hangover. If I solve a puzzle with my lunch, it helps me to dose off to a peaceful nap.
Prasamsa2 years ago
same quadrant here means??how much area is considered as a quadrant??im confused in this step..
2 years ago
A square of 3x3 is a quadrant or grid. There are 9 of them in the puzzle.
2 years ago
Prasamsa,
I'm pretty sure he just means one out of the nine "boxes" in the main grid. I guess if "nona-" is the prefix for "nine", then he'd be talking about a "nonadrant". LOL
I came to this web site hoping to find out where my thinking is muddled when I get down to the same three possible numbers in any box, column, and/or row. I can't seem to figure out how to further eliminate possibilities. If you - or anyone else reading this post - can help, I'll check back!
Thanks!
Granny H
2 years ago
You shouldn't end up with a solution where there are only three boxes left, and you have the same three numbers for each of them. In that case, it seems like you could allocate the numbers as you please, as any will be a "right" answer.

If you just have a place where you have the same three numbers appearing in the same box, column, and row, but there are other bits left to solve as well, try to work on other parts of the puzzle. Somewhere in the puzzle, there's some way to eliminate some of those numbers. You might wind up coming at it from a really backward direction, but it's there.
2 years ago
Thank you for trying to help, PurpleKat.

I think I stated my problem poorly. Let's see ... What I meant was more like what you say in your second paragraph. When I have several "other bits" of the puzzle left here and there, and the same two or three numbers are the only ones available for all of those little boxes, I don't know how to eliminate without guessing. And guessing NEVER works for me! I'll come back again when I bump into a good example to give you.

Again, thanks!
Granny H
2 years ago
If I could see a puzzle with the problem on it, I could tell you how I'd go about solving it, assuming I knew how. I'm not a master at this -- just someone who loved logic problems as a kid. :)
rjvkshl2 years ago
You have painstakingly described a beginner's method which I had adopted a few years back. I prefer to 'soften up' the puzzle as much as I can, before embarking on this method, which I have privately termed as the 'slog phase' of solving the puzzle.
How do you soften the puzzle? Select a pattern of rows and columns which have the highest number of digits. Proceed to tackle them in descending order of number of digits.
Once you have selected a row or column with the highest numbers, attack it in an oblique manner. For instance, if you are tackling the upper row, you may find a number repeated in the sub-rows. Now try to fit that number in the 3rd row by looking into the sub-columns.
Attack all 3 horizontal rows and 3 vertical columns turn by turn, systematically in this manner and you would have softened up the puzzle a bit. Repeat the entire process and you'll get a few more discoveries, maybe one or two.
Once the puzzle has become soft, you can start the slog phase. It is NOW that you need a pencil & eraser.
Don't make all entries (from 1-9) in every box. You can straightaway start with the missing numbers ie possibilities.
Once you have entered all possibilities, start looking for singletons - row-wise, column-wise and grid-wise. Having caught singletons, erase the corresponding possibilities row, column and grid wise.
Now look for pairs, triads and even in a few cases, 4-digit patterns. Having identified a pair, triad or a quad in a row/column/grid, proceed to erase the numbers in the remaining possibilities.
Don't forget, once you have discovered any digit, always look obliquely in horizontal as well as vertical directions to fit it into a suitable blank space, as I have described in para 3 above. It helps.
Ultimately you will reach a point where all the solutions cascade into view.
bobc40122 years ago
While this approach is good for solving the simplest of Sudoku puzzles, it is only a start on the more difficult puzzles. Once you have "captured" some of the squares in the more difficult puzzles, you will find a number of squares where it looks like "guess work", when it actually requires "advanced techniques" to solve the puzzle There are techniques like "X-Wing", XY-Wing" an even "multi-color" solutions, to name a few. The site http://www.angusj.com/sudoku/ has an ".exe" file that can be downloaded (free) and is useful for solving hard Sudoku puzzles (it has a decent help guide along with googling some of the advanced solving techniques). When you get stumped, you can click on the "hint" button and it will indicate a square and the technique to derive the answer for that square. In certain instances, the technique isn't always straight forward and googling will help clarify.

One thing that is the case with most Sudoku puzzles is the symmetry. You notice the symmetry at the start and as the solution proceeds, you will notice a symmetry in the solution. In this case, this is where I agree with Eddie Chong on the "beauty" of the puzzle. What I detest most are those puzzles where you are reduced to guessing (rather than using logic and reasoning) which of 2 or 3 numbers to to choose in 2 or 3 squares and use "trial and error" to solve. This is typical in most Sudoku magazines that have "extreme" or "brutal" puzzles. To me, you should never have to resort to "trial and error" to solve the puzzle. As Sir Arthur Conan Doyle had Sherlock holmes say "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."
10T3 years ago
It's great to see all the comments from so many dedicated puzzle solvers! I just started Sudoku, so using this technique along with tips from other websites really helped me to learn the methods quickly, for example, solving by pairs and triples. Thanks for creating this website.
broper3 years ago
very nice
good job on you ible but one thing i want to menchon dosint this methoud take the fun out of sudoku? i tryed it on a hard puzzel and i didn't feel like i completed any thing when i was fineshed.

nice way to do it but it takes the fun away
3 years ago
I disagree... if you used this method on truly hard puzzles, even with "notes" (as I call them) they can be difficult. A puzzle is a problem meant to be solved, this happens to streamline the process. Goodluck :)
3 years ago
yes thats a good point but i think it makes it to easy i mean i like doing hard puzzles and i enjoy the chalenge but if i do it the way you suggest i think it makes it two easy

ps

i think the methoud you made is a good one and i have nothing roung with it
it does its job it makes sudoku easy ( :
3 years ago
Try cross words, Seems like you'd have plenty of challenge with those :)
3 years ago
you have a point

my dad is always sayin' that it has to be chalenging
3 years ago
tx ( : i think the way you do it is awsom. and smart that you came up with it

WillyTheKid3 years ago
I try to get everybody I know to play Killer Sudoku (or Sum-doku). So much more of a challenge than even the hard level sudokus. but still there is a system.. always a system, even if it is complex. no need to guess!
bjohnson13 years ago
You should only use a pen and no eraser! That is how to do Sudoku.
siamonsez3 years ago
I'm sorry, but this method feels like cheating to me, and at the very least defeats the purpose of doing the puzzle in the first place. You can resort to this method once the majority of the puzzle is done and you are totally stuck, but in my experience, you only get stuck because you either missed something obvious, made a mistake, of are trying a puzzle above your skill level that requires the use of techniques you are not familiar with.
Anyone that wants to learn to sudoku should avoid this technique because you will learn nothing by finishing puzzles this way and therefor will never be able to progress on to more challenging puzzles.
3 years ago
I really cannot see what the difference is? The only difference i see is that this is systematic and the other way of doing it is less systematic.

This is nott a game of memory, it is a game of systematically solving a grid puzzle.
3 years ago
The difference is that if you rely on this method you will never get better. Once you start doing sudoku puzzles regularly, it's not just about completing them, but doing it quickly; anyone can finish , what makes it fun is doing it better.
3 years ago
The answer to this problem is not as straightforward as that :) It all depends in which way one views "getting better" and "fun".
3 years ago
Maybe I have a bit of ADD, but once something gets monotonous, it gets boring. So, for me, pushing myself to do better is what makes it fun; therefore, the two are interchangeable.
3 years ago
right !…
100% !…
I'm not against cheating, but it's got be worth it. In this example we didn't even try to begin to play sudoku : may as well play pencil and eraser on any piece of paper.
anyway… the example given is so basic that it's not even worth taking the pain to write all those numbers down : this sudoku can nearly be solved by itself !!!…
3 years ago
This is simply a visual representation of what your thought processes when finding the answers. The only reason it's easier is that you don't have to remember everything you've done. Of course more advanced puzzles will require more advanced techniques but a lot of those techniques are based on logical trial and error, which could be considered cheating just as much as this method.

This is a good way to start if you are learning sudoku, eventually you will learn the patterns and you wont need as many pencil marks. Besides if you are just starting and need this method to solve a puzzle there is little chance that you will know or be able to use the more advanced techniques.
Panta Rei3 years ago
Seriously, I've always thought that everyone in the world solves sudoku this way , just omitting the numbers already given in the related rows instead of writing them, and so I do. So...how the others do?!
bjarvis23 years ago
I'm creating a C++ sudoku solver for practice, and this algorithm looks great for a square 1 starting place! There are probably more efficient methods for automation, but this algorithm is a systematic approach to what a human does, albeit (or although, depending on who you're talking to) very explicitly stated and discrete.
zheyman3 years ago

Notice Row-3 has 2 squares whose only values can be 2 or 3. That means if one square is a 2, the other must be a 3. So no other square in that row can have a value of 2 or 3.

Removing all of the 2's and 3's from the other candidates in the row reveals the yellow square to be a 5. That forces the last square in the row to be a 4 and the remaining square in that row to be a 9.

Use this technique and more at http://www.SudokuByZol.com.  It has the best pencil mark support of any Sudoku website.

aniketpatil3 years ago
Thank u so much for this technique.
chinmoy samant3 years ago
he thanxxxxxxx it was just mind-blowing'awesome............................
cat203 years ago
hey thanx ,this is wat exact solution i was searching for
eddie chong4 years ago
No doubt this procedure will give you the solution and the puzzle is "solved" as
quickly as possible. The uninitiated to this rather 'mechanical' method will be impressed, like I was a moment ago having followed your step by step approach.

I would not, however, resort to this 'protocol' The hallmark of Sudoku puzzles
lie in its intrigue and its beauty as a puzzle to be held in awe, respect and a
challenge to be taken. I often complete the day's puzzle and enjoy my successful
attempt with self satisfaction and look forward to the next day's fresh gauntlet, however long a time it took me. In Sudoku the journey is as enjoyable as its destination.

EddieC
3 years ago
Don't mechanically enter them all first. Solve as much of the puzzle as you can. Only then resort to this method. Most puzzles of any difficulty require Pencil Marks (as they are called) for at least some of the squares.

If this method is too tedious for you, try out the many automatic Pencil Mark features at: http://www.SudokuByZol.com
3 years ago
@eddie chong While the idea that sudoku is good for mind, body, soul and spirit is nice. the interminable headache you get after trying to solve a more advanced puzzle without doing this is a slight problem for some of us, or at least myself due to a lack of an endless supply of patience. However this is a method that should be used as a last resort before finding the answer key.

and to the author thanks for the tips.
nanma3 years ago
solve the columns with the most numbers first.
The fourth column going down.
has 6 numbers
1 3 4 9 2 5

so 6 7 and 8 are missing.
so the box between the 4 and 9 must
cant be a 6 or a 7 so it must be an 8.

nanma3 years ago
use dots instead of numbers. and try to keep it to a minimum of 2 dots. per
blank box.
atomic12693 years ago
I guess this is the "beat it with a sick" method, so to speak. I find it easier just to use some real simple rules so you don't have to look at all the different possibilities. This site explains it pretty well - http://a-how.com/archives/247
3 years ago
Pencil marks (as the potential candidates are called) are useful after the simple rules have been exhausted. For harder puzzles, they're almost impossible to do without.

For fast and efficient pencil mark support and free online sudoku puzzles, try http://www.SudokuByZol.com
Pentacle4 years ago
Well, it definitely works! It just takes a long time to write everything in and erase it.
4 years ago
Too tedious ?? Just one click fills them all in at http://www.SudokuByZol.com

Comprehensive and extremely efficient pencil-mark support and user interface.

Play never ending supply of puzzles (5 Levels), or enter your own puzzle using the Solver link or button.
camcuru4 years ago
This is the very first step to solving Sudoku puzzles, but it will only get you so far.
This method will NOT be sufficient to solve 197 of the 200 Sudoku puzzles found in the Extreme Sudoku book by Antoine Alary ( sudoku.alary.org ).
Get that book if you want a real challenge, it even has the pencilmarks pre-printed in half of the puzzles, as most of the advanced Sudoku solving techniques make use of them.
4 years ago
You can enter the puzzles from that book into http://www.SudokuByZol.com and solve them online using the techniques here and others.

Efficient keyboard or mouse user interface that won't get in your way.

Just click the Solver button or link.
ToddisI4 years ago
Pure amazingness!!!!
KLUTZYengineer6 years ago
Is it plausible that this "system" not work properly.
If not - Go to websudoku, click "Evil" and attempt to solve 2,200,342,256, it should appear to be the first one it shows, if not search it. It would be very helpful if anyone is capable of solving this sudoku. Reply to me through my inbox if you have conquered the EVIL SUDOKU.
4 years ago
It is solvable You should download simplesoduko It will let you design your puzzle and teach you so many methods such as xy wing, coloring, etc. Answer is 849635721 271849653 563127984 714352869 382916547 695784312 157463298 436298175 928571436
4 years ago
Nice puzzle.  My SudokuSolver program, which uses basically the same method proposed by this article, was unable to solve it, or even get very far, on its own.  I only had to solve a few numbers manually to get it there, though.

6 years ago
does not* AND ALSO REPLY HERE.
5 years ago
i solved that puzzle just now...took me 44mins and 8 secs but i did it...the tricky part was solving the top and bottom quadrants in the middle column of quadrants
5 years ago
hang on...i jus realised i solved it on easy....now i feel embarrassed (o_o)
6 years ago
2,2etc. is solvable. My method is to go through putting a one in every square that isn't blocked by a one already in that row, column, or quadrant; then repeat with two, then three, etc. When you finish the nines, any square with only one number in it is solved. This should eliminate some additional numbers. When you have gone as far as you can with this, look for pairs of squares with two matching numbers. Example: if two squares in a column have 1 and 5 (only) in them then no other square in that column can have either a 1 or a 5. This can also be done for three squares that share the same three numbers. Finally, if a number appears multiple times in a quadrant, but only in one row or one column in that quadrant, then it cannot be in any other quadrant in that same row or column. I hope this helps and that I haven't only told you things you already knew. Thanks for the heads-up to this puzzle. It took me about a half hour to solve and was a lot of fun. Good puzzling to you.
zheyman4 years ago
This solution is called pencil-marking.  You can automatically or manually create these at http://www.SudokuByZol.com .  Best pencil-marking support of any online website.

Unlimited puzzles.  Even let's you enter and solve any puzzle from a book or magazine.  There are hints and you can print your puzzle to take with you.
malallah4 years ago
Ive do it .takes 1 hour 2 me
ajjardin4 years ago
Got to love Sudoku. a Great way to build the mental. learn, practice, and grow. great explanation, its always interesting to learn different Sudoku Strategies
sachinyelmar4 years ago
Due to this sudoku is easy to solve......
zheyman5 years ago
For some great information on solving techniques that don't include guessing see the wiki at: http://sudopedia.org/wiki/Main_Page

For a great free online Sudoku experience try Sudoku By Zol: http://www.SudokuByZol.com
zascecs5 years ago
SOOOOOOOOOOOOOOO AWSOME!!! Thanks!
loganbell5 years ago
This man is taking credit for your work i believe... i saw this on instructables a while ago, and just read this article:http://www.usatoday.com/tech/science/mathscience/2009-03-15-sudoku-secret_N.htm?loc=interstitialskip the instructions can be seen here: http://www.ams.org/notices/200904/rtx090400460p.pdf its the same concept, right?
zheyman5 years ago
For those who think this method is too tedious, try this.

Solve as much as the puzzle that you can before it's more difficult. Then fill in the grids for the remaining squares.

Plus, only write the numbers that haven't been eliminated.

Sudoku By Zol makes this easy. For beginners, one click fills all of them in. Advanced users can do it manually.
zheyman5 years ago
These are called Pencil Marks in the Sudoku community. They narrow down the candidate values for a square. Here's an example.

Let's say two squares in the same row can only be a 3 or a 4. That means you can erase a 3 or a 4 from all other squares in that row's pencil mark values. That further narrows the candidates.

If you don't like all the work of filling in the pencil marks, try http://www.SudokuByZol.com.

Unlimited puzzles with automatic (and manual) pencil mark support. It has the smoothest and most efficient user interface of any Sudoku site out there.
bullitje5 years ago
this is allot of work for something simple as a soduku. I just use a number by number approach , write on your paper , the number 1 trough 9 , and start with 1 , and follow trough, use the 2 obvious ways of elimination, (1 elimination form rows and collums, 2 elimination from square) and then you should solve all the easy one's. when you completeted a number , you scratch it away. the hard ones , you need to figure out 3 more ways of elimination. i can't describe them , but they use simple looking ahead. And about you have to pick a random number that is nonsense , you just have to look far ahead enough to see the consequences, with some practice you can look ahead 5-10 steps. There are chances that you see "pairs" of numbers , then scribble down the pairs, so you remember them quickly when you fill in 1 of the numbers.
sudokulover5 years ago
thanx a lot...I always solved the easy ones pretty quickly but fumbled invariably over the tough ones.. This would help me a lot... Thank you very much
Hey! TY with this. This is much a help. And easy but longer to work. 8-)
dombeef5 years ago
WOW 249,389 views in just over 3 weeks !!!! Congratulations! :D This is the best Instructable I had ever seen! ( No offence to other Instructables)
zulhanif6 years ago
yea sure. its smart. but wheres the fuun? its a nice strategy, but id rather think than going through all the work. :P
Johnsons on fire6 years ago
won't this take even more time than usual with all the writing?
wenpherd6 years ago
mendoza336 years ago
Isnt this more work than it should be....?
PKTraceur6 years ago
using this algorithm (can i say that?) to a point, wouldnt it be easier to just start in the middle? The you could easily get the rest of the puzzle quicker...
m_yazhini6 years ago
it it is nice
gpj6 years ago
I just tried this for an Evil rated sudoku. As other comment said, you don't get a solution for this degree of difficulty. However, I think this works as additional step: 1. After step 10, lather, rise, repeat you may not be left with single numbers. Find the group with just two squares empty, each with two numbers. At this point you have to guess. 2. Copy your paper exactly as you have it now, so if you guess wrong you can try again. 3. Guess the pair of numbers for the two squares. 4. Continue with the method above. If it works, you guessed right. If you guessed wrong, it will go wrong. If you guessed wrong, go back to your copy, guess the other way. This is still a mechanical solution, no thinking required. There's a lot of paper and writing though, I did find this rather time-consuming. However, I did solve the evil sudoku much faster than doing it my usual way.
sudoku2day.net6 years ago
good method
Plasmana6 years ago
LOL! that is kinda like 'cheating' to me, but that would be very good for the sudoku beginners...
=SMART=6 years ago
HOLY C*** DUDE!!!<br/>249,389 views in just over 3 weeks !!!! congratulations :D<br/>did you post this on other forums and stuff?<br/>
Lo2a6 years ago
Thnx alot, i used to think wen i solve sudoku but now, i won't
mouse236 years ago
When writing numbers (1-9) into the vacant squares, one could immediately omit the numbers that are given in the "related"rows. This saves having to come back later and erase. The numbers that are written down should be placed in the appropriate position in the square as in the examples, just for visual aid.
=SMART=6 years ago
nice way to solve it ! i still prefer xbox for entertainment :D
catsaway96 years ago
There are some more strategies you might want to try at this website: http://www.sudoku-strategies.net.
khokleut7 years ago
I am not "great" at Sudoku...been working on them for a little over a month and have gotten up to the 5-star puzzles. My problem is when I get to a point where I have two possibilities per box, across the row, and I can't eliminate any. Any help when you get down to the last few and there's no way of going further without taking a guess?
7 years ago
I have "solved" puzzles with more than one solution, but I think they are rare. Generally when this happens, the same two numbers are possible interchangably, in four of the nine quadrants. Usually, though, it only seems that the numbers are interchangable. I "pick" one number and trace the effects through the remainder of the puzzle. You will either run into a roadblock (so the other number is the right one), or you can go all the way and solve the puzzle. Use pencil or change colors for this "what if" scenario.
6 years ago
6 years ago
for all valid sudoku puzzles there must be only one possible number for any given cell. I guess you might want to check out the algorithm from How to Solve Sudoku . You might also want to use the pencil feature at Pencil Sudoku
codeshepherd6 years ago

For those without a facebook app. Colorcoded Sudoku
I came up with a similar method to this when I first came across sudokus. Look for the cells which have most 'clues', then write all the numbers which COULD go in those cells, small, at the top. When you find another correct number, cross off small numbers vertically, horizontally and in the same 3x3. When only one number is left in a cell, write it in large. This method starts off slow, but accelerates as you get near the finish.
unit846 years ago
This was done with an incredibly simple puzzle, and the process is a much lengthier method (instead of looking at every single possibility, just look for the ones that fit). For those who argue against sudoku for crosswords, this method would be comparable to looking at every single word that fits in a space and fullfills the letter requirements i.e. Looking at three letter words that start with 't' and end in 'e'. tae, tee, the, tie, toe, Plus using the hint, i.e. stalemate. Answer: Tie (Obviously a simplified puzzle word, but I think my point gets across, and I thought it was only fair considering this was an easy puzzle to solve as well) Genius method? Not really Both sudoku and crosswords are both challenging if you pick difficult puzzles and think. I invite everybody to do so.
tim5496 years ago
Free open source sudoku puzzle solver and creator at PseudoQ
codeshepherd6 years ago
Free online color coded sudoku puzzle solver with timer, pencil, graph at Sudoku Solver
J_SCAP7 years ago
genus
7 years ago
My words exactly.
7 years ago
hmm, i would have said genius...but yours is good too.
6 years ago
He he yeah, I wouldn't consider Sudoku a group of species...
YummyPancakes6 years ago
Yeah, I have heard of this method. It also works better if, when you're writing in the missing grid, that you just go ahead and not put in the numbers that can't logically fit in. It should save erasing, resulting in a prettier solved puzzle.
lavenderblue6 years ago
I have been searching the web for solutions to Sudoku. This is a very easy to use, systematic way. Thanks! I tried it last night (my very first attempt to do a Sudoku puzzle) and solved it! I did make some errors and erased a few numbers wrongly. Just wondering. Say for more difficult puzzles (my friend said he solved one in which only 9 cells were filled in), what do I do if after using your method, there is no clear answer? Then itâs just trial and error? Is there some âformulaâ? My friend says he uses a formula but refuses to tell me what formula it is.
ginnyrae7 years ago
6 years ago
Generally speaking exactly the same. If you think of the letters and numbers as "symbols" rather than numbers and letters they're exactly the same. Shouldn't be any harder, you just have to find out which letters are being used.
7 years ago
oh my god, they have letter ones? how does that even work,i understand your plight
7 years ago
course find a nine letter word and substitute letters for numbers
7 years ago
(removed by author or community request)
7 years ago
We have a "be nice" comment policy. Please be positive and constructive with your comments or risk being banned from our site.
7 years ago
Hey trebuchet03 you get around! I've been just casually going on this site for about... a year? maybe two. You're everywhere, it seems.
7 years ago
replace the letters with numbers and treat them as ordinary sudoku puzzles. I actually tend to do the opposite because my brain always wants to do math when it sees numbers and pattern-finding when seeing letters.
thanx a lot...I always solved the easy ones pretty quickly but fumbled invariably over the tough ones.. This would help me a lot... Thank you very much
dandym7 years ago
Great idea. I absolutely hate Sudoku. Now I can solve them easily and show everybody what a pointless exercise they really are. Crosswords are still the best, they improve your vocabulary and you actually have to think!
7 years ago
I don't hate sudoku, but there's no doubt that crosswords serve an intellectual purpose...I suck at crosswords though :'(
7 years ago
THANK YOU!!
TedTheRed7 years ago
Nice idea- I set up an excel sheet which adds the rows and columns and has squares large enough to enter all possible numbers in the blank squares. I'll have to see if I can redesign it.
jun377 years ago
this is great! i used to solve it the tedious way. thanks!
ikenho7 years ago
very good i will trie to use this method in the future thanks!!!
Aerospaceman7 years ago
Sudoku is too cool! When solving each puzzle think of the numbers like chess pieces, like rooks. No same number can be in the same row or column. For example, see the sixes in the middle blocks, left side middle right side, and the right side top right? A six will only fit in the middle block, bottom center. A logical process of elimination. I love solving the meanest Sudoku's I can find.
Meragness7 years ago
great method! by saying this is the easy way, do you mean that there are other ways to do it? I do it with process of elimination but I hate writing all over it, so I just keep track in my head ( this is why I mess up so much)
arthaha7 years ago
good method, but it takes way too long...
Mike Cortes7 years ago
lol, i havent tried it yet but it is a brilliant idea.
egreen7677 years ago
crap! I cant get it to work. I probably screwed up somehow.
jrandall017 years ago
Another tip is to look for "completed threes". Using the sample puzzle, suppose you figured out that the center spot in the top center cube is a 7 (1-7-4). This is a "completed three". The 8 in the top left cube tells you that an 8 must be in one of the two empty spots in the top right cube because there cannot be an 8 in the center row of the top right cube, and there can't be an 8 in the bottom row of the top right cube because that would mean there must be an 8 in the top row of the top center cube (where the "completed three" already fills all of the spots). Similarly, the 2 in the top left cube means there must also be a 2 in the top row of the top right cube (same logic). since there are only two open spots, and they must be occupied by an 8 and a 2, and since there is a 2 in the right column of the bottom right cube, the top right row must read 2-5-8. Use this logic for "completed threes" in rows and columns. Happy puzzling!!
Ricky282697 years ago
You missed one! The left-middle sub-box inside the middle box. It's eliminated to an 8. As much as I hate to be picky, the "Add Comment" button was just too tempting. Sorry.
zoltan89057 years ago
Hi Everyone!

I am ZoltÃ¡n from Hungary .. but I suppose no one cares about where I write from...

I just want to tell you all that I have a Sudoku solver (by 'have' I mean I did it by myself) and it can be reached here:

http://www.templom.atw.hu

However the program and the website both have a Hungarian language it is not that hard to use it.
If anyone is interested in using it, just write to me an e-mail (templom@atw.hu OR zoltan8905@freemail.hu) and I will make an explanation on how to use it ('cause it has 'hidden' functions).
Anyway, if I get a request I will kindly write it here or just send it in e-mail.

Good luck and great pleasure to You!
conductor_AJ7 years ago
And PENCIL erases off my nintendo, too.
Isn't this just how it is done? When I first picked up a soduku, I just naturally started doing it this way... I thought everyone did. Guess that's why I thought people who were obsessed with these or said these were hard were stupid...still do.
7 years ago
People who say Sudoku is hard are stupid? I could say the same thing about anyone who actually thinks this method will solve anything but the easiest Sudoku puzzles. But I don't actually believe that and I don't call people names, so I won't. This method is great for the easy puzzles and is a wonderful way to get started, but enter the following into a grid and try to solve it with this method. I'll use 0's for blanks to help space things out. Note that this is an intermediate level puzzle so they get even harder than this... 020000400 300009005 000050010 076004000 000000807 009003050 800400300 000000092 900710000 This isn't a personal challenge and I'm not trying to belittle anyone. I'm just trying to insert a reality check here. Sudoku puzzles can be EXTREMELY difficult. An experienced Sudoku solver should be able to solve the puzzle given in this instructable without making any marks other than entering the correct numbers. It's a extremely easy puzzle. Good luck to all.
CVJX7 years ago
I have actually used this method \. it's called "candidates". I saw this in a book and you still have to think when you do this. anyway, nice instructable and it was the easiest way for me too.
gamerx3657 years ago
this is what i do... i go on WebSukokuand in another window goto SudokuSolver and see how fast i can get it done. Sudoku solver actually knows how to beat any sudoku, its so cool
Nuyorican7 years ago
What is the point of trying to solve a sudoku if you don't use your brain. What is wrong with admitting u can't solve a sudoku or take you ten hours. I'll be the first, I can't solve a sudoku. See that was easy. I'm not big on puzzles anyway. I have bigger real life puzzles to burn braincells with. Same goes for cheatcodes. What is the point?
7 years ago
I guess you're not big on spelling or grammar, either. Be nice don't post, dude.
7 years ago
What's wrong with an instructable detailing an automatic method for solving practically every sudoku puzzle without thinking? Some people knit, others solve sudokus.
amcq7 years ago
The "first" step should be to scan the board looking for the OBVIOUS numbers that go in certain spots. For the example above, look in the left 3 quadrants. For the top-left, the number 6 is in the middle column. For the middle-left, the number 6 is in the right column. This means that for the bottom-left, the number 6 HAS to be in the left column. For that left column, the only possible locations are the 2nd and 3rd cells. You then scan both of those towards the right, and find that one of those rows has a number 6 already. This leaves only one cell for that number 6, the bottom-left cell. You can continue this logic for all columns and rows, and all numbers, to find numbers that can be immediately filled into the puzzle. After these are filled in, there's drastically less empty cells, and makes the process described above much faster.
karen6087 years ago
As one who often got incredible headaches from trying to do a puzzle, I really am glad you have a system. I can do a few and not be left out in my crowd where they think this is really IT. Good job. Very logical.
mopeycalliope7 years ago
The purpose of puzzles is to think and analyze. that is where the fun lies and where the satifaction comes from when you solve it. but i guess everyone uses their own strategy.
Unforgiven_God7 years ago
There's also a 6 in that quadrant...you missed it (or you didn't post it) :(
Unforgiven_God7 years ago
Well, actually the whole time you are doing this you are thinking, just not as much as you usually would. :D just sayin
I don't see the point of solving a puzzle without thinking. Why do a puzzle if you do not want to think? Sorry, but I say this is cheating.
katesisco7 years ago
When I first started Sudoku I didn't have the 'whatever' to remember and figure all this out in my head; my Dad taught me chess but I couldn't keep the 'game ahead' moves in my head either. So, what I did on my own was to do exactly this; put all the numbers in all the open squares, later I realized that I didn't have to make this much work for myself and just filled in the 9-box squares with what ever numbers were missing from the 9-box; example a 9-box with a 3 and a 6 so I penciled in 1,2,4,5,7,8&9 in all the others squares in that particular 9-box.

Then you erase all the numbers that are dups from each column up and down; works great if you don't make a mistake and think you only have one 2 and overlook the second one so the puzzle goes bad. Or if in erasing you don't erase all the dups of the original number. Other than that works great; 90% of the puzzles solve this way but then again you have the top level of difficult (fiendish, or some such) and you come out with no single number to work off of.

This usually leaves you stuck with a choice of two pairs the same and you have to pick one and go from there, it requires you to make a copy since you just might have picked the wrong one and have to do it with the other choice. I do not like these, not because they are harder but because they don't flow as well. I have tried to find a solution that would let me continue without guessing but even looking at web sites I have not been able to find one that would let me go on. And I do understand that if there are a 6&9, and a 6&9 say in a nine box that right there we have the six and the nine so all other 6&9s in that particular 9-box can be erased. This also works if they are line up in a column either way. And I do know that a 1,6&9, 1,6&9, and a 1,6&9 in a box or column allows all other 1,6&9s to be erased.

But occasionally I wind up with a puzzle that doesn't have even these features and I am stuck---------anybody?
JuliaGlass7 years ago
I do this mentally. I don't really care for sudoku, but I can solve the puzzles fairly easily if I have to.
jerrylives7 years ago
This takes all of the thinking out of the puzzle. This makes it a write numbers and erase them exorcise.
uzerzero7 years ago
I've never actually used this method, well, not entirely. I use process of elimination, but never wrote it out. I kept it in my head, partially because we do it in the newspaper at work, and we almost never have pencils available. By the way, I hate the sort of mechanical pencil you have in the first picture.. they're one use only pencils. Can't refill the lead.
delgaudm (author)  uzerzero7 years ago
That's funny that you don't like the pencil. I LOVE them, have for years. In fact, I wrote a <a href="http://www.epinions.com/content_77821808260">long review</a> of the sharpwriter about 4 years ago on Epinions, and still use them every day.
7 years ago
After reading that, I decided to give them a chance, and I'm actually starting to like them. The lead is very thick, and is great for drawing, and it doesn't get all thin like some other leads do. Perhaps one day I'll learn to love them :)
beelzebub1238 years ago
that is that exact technique on sudoku in brain age
7 years ago
actually, it's the reverse use of the tactic i use in brain age; i focus on one number at a time and place a small number in each box, rather than fill every single box with small numbers. Also, in brain age, you can't make any marks on the squares that already have numbers printed in them
inevitable_chaos7 years ago
Probably the first strategy i used, great minds think alike. Eh?
7 years ago
i used this when i did it. its the easist way, i never got it when the maths teacher put it on the board, but i talked to mr wood and he showed me this
egonx7 years ago
egonx7 years ago
... and "Naked Pairs, Triples & Quads", "Hidden Pairs, Triples & Quads", "X-Wing" and "Swordfish"
egonx7 years ago
It's not always that easy. You'll need more elimination tactics then "Singles" and "Hidden Singles". Such as "Locked Candidates" (two kinds)
dkfa8 years ago
WTF i dont get it!!!!it doesnt make sence!!
Caratje8 years ago
Hi All, also I like sudoku, but hate the trial on error approach and the same holds for too large/fuzy sets of strategies. I read a story about permutations which so far I found most interesting/new for me. This theory is based on that both total sum and multiplification of all numbers in each row/column/block is fixed. In stead of 1-9 the first 9 primes are used and if you then subtract the knowns from this fixed number, you can in that way specify the unknowns. Curious if somebody used this notation method in his solver so far? - and found interesting things... - And how about designing new sudoku puzzles yourself? (how do you know the solution is still unique, but by trying to solve it yourself?!?!?)
8 years ago
WHAT! lol i dont kno half of wut u just said u have to teach me man pl\z lol......i will be very greatful
Kevvixx8 years ago
I really liked this strategy and will try it myself.
xallie8 years ago
haha, very nice. gj!!!
Robin_ala8 years ago
I'm sorry, but I'm running into problems. Unless I've done something wrong, I have, in a couple of puzzles, arrived at a place where I can find no single ocurence of a number in either a row, column, or quadrant. Every number appears at least twice in a respective area. MUST there always be a situation where there will be a number ocurring only once in the various areas? Robin
8 years ago
Ignore this message. I didn't realize there were more comments than I first say. Reading through them, I've got my answer. It's working nicely.
TheCheese99218 years ago
yea my math teacher made us do some i couldn't figure it out this is so awsome
onTheJDAR8 years ago
I like this strategy. Makes it look easy!
8 years ago
Yes.This will really help me. I prefer to play online on my computer AND on the go with my mobile connected to he community www.worldsudokuleague.com. Unfortunately I can only put one figure in the mobile game, but the online game gives me a possibility to use with your strategy, except from drawing lines and circles, but it should not be to easy... :-) I will now move upp on the ranking. So thanks for that. Holger L
8 years ago
This strategy is exactly what i have coded up on http://sudokuhelper.kevinpluck.net

It really does make it easier!

You can solve the above puzzle by following this link:
http://sudokuhelper.kevinpluck.net?g=83f3dcd50b4306951724ee7e0a9b4fb4'
8 years ago
Actually you get the solution for the above puzzle in about one or two steps with sudokuhelper. Here are some more difficult puzzles which take about 30 or so steps:

http://sudokuhelper.kevinpluck.net?g=3a70caa2d32282d27ce1cfb455e4ef93
http://sudokuhelper.kevinpluck.net?g=2ea40d2fb5b28c62e72945bdeeb38549

More here http://www.kevinpluck.net/forum/viewforum.php?f=1

Enjoy!
lulu2345677268 years ago
THIS IS GREAT ! IT USED TO TAKE ME ABOUT 30 MINS TO COMPLETE ONE! NOW ITS JUST 10
Yakeyglee8 years ago
Wow. I guess you proved me wrong. I never would've thought that you could solve sudoku without thinking. Loses all of the fun though. I wish you could do that with crosswords :P.
Tushar8 years ago
i have a sudoku solver, which runs on microsoft windows, if you need, write me at tusharkd@hotmail.com
Try this free online sudoku game too:
http://www.free-sudoku-games.net (it allows you to get a pdf for the puzzle as well)
lianoir8 years ago
sigh, cant post it with _ as neutral's xxxxxxxxx 5xxx8xxx7 x9x4x1x2x x3xxxxx1x xxx659xxx x2xxxxx4x x1x2x3x5x 6xxx7xxx8 xxxxxxxxx
lianoir8 years ago
This method does not solve every logical sudoku. e.g., try this one: _________ 5___8___7 _9_4_1_2_ _3_____1_ ___659___ _2_____4_ _1_2_3_5_ 6___7___8 _________ (but it does solve the easy ones, but he, there's no fun in there anyway)
mewantbigboom8 years ago
if only i learned this earlyer
ravi8 years ago
I was very much impressed with the method.I am glag that I have visited this best of my career site.I will be in touch & sent my ideas also soon.I got this site address through Times of India paper,where I am daily going through this site address page & noting the sites to be seen on my digital dairy.Thanks once again Ravi Gavde
netbaby8 years ago
"Lather, rinse, repeat". Hilarious! I loved it! Wellwritten and fun. Me and my mates have been working on our own Sudoku Solver to complement our existing little hybrid web/mobile sudoku project, World Sudoku League, available at www.wsl.nu
WolvesOfWar8 years ago
is there an otherway? btw, i(with my dad) wrote an exel sheet, wich solves every one(by this methode)
zer0vector8 years ago
I'm confused by the people saying "that makes it too easy". Isn't this the way to solve sudokus? If not I must be doing something wrong. Do other people guess or just work on very easy puzzles? I only enter numbers when I'm sure that they go in the square (ie I've eliminated all other possibilities), and on any ones harder than medium (ranking based on www.websudoku.com), using a notation method is necessary in my opinion.
Amy Haarth8 years ago
I can't thank you enough chris. You are great. I have started doing the sudoku puzzle lately and am only familiar with the trial and error method of solving the puzzle. But there has to be a logical way of doing it. Does one look for a pattern of numbers or a grid or what? Can you take the time (whenever at your convenience) to explain how you got the answers. There's no hurry. Thanks
Amy Haarth8 years ago
Hi all, I just signed up with this site. I am writing this in desperation. I need a solution to a puzzle as soon as possible. Somebody has challenged me and yes I am cheating. It's an evil sudoku puzzle. This is how it goes.B are the blank cells. 3BB 49B 1BB 2BB BBB 7BB B6B BB7 BBB B26 BB8 B9B 5BB BBB BB7 B3B 9BB 84B BBB 8BB B3B BB3 BBB BB8 BB4 B51 BB9 Please respond ASAP. Thanks in advance
8 years ago
GRID: {3}{7}{5}{4}{9}{2}{1}{8}{6} {2}{4}{9}{1}{8}{6}{7}{5}{3} {8}{6}{1}{5}{3}{7}{9}{2}{4} {4}{2}{6}{7}{1}{8}{3}{9}{5} {5}{9}{8}{2}{4}{3}{6}{1}{7} {1}{3}{7}{9}{6}{5}{8}{4}{2} {6}{5}{2}{8}{7}{9}{4}{3}{1} {9}{1}{3}{6}{2}{4}{5}{7}{8} {7}{8}{4}{3}{5}{1}{2}{6}{9} Hope this helps. One day I wrote a brute force solver that can handle the most evil of solutions :) Chris
jshields8 years ago
I tried out this method but by the time I had filled out half the boxes with 9 numbers in each my hand was already hurting and it made me not want to even bother to finish. Then when you try to erase a tiny little number written in pencil others around it smear. It just seems like alot of prep work and it's alot faster to use other methods of solving the puzzles, plus this takes the fun out of it.
gormly8 years ago
Um... you poeple will lose to me everytime if you follow this method.. my quicker method is to only fill in the "tic tac toe" with numbers NOT in the veritical row starting from the top. For instance the only numbers in the tic tac toe grid for position are three and nine.. there is NO need to put a full on grid in the first position, it is a waste of time. Fill out the first row grid and follow the pattern, once you have the first row.. the rest is cake. even faster than this listed method.
Henrietta8 years ago
Where's the fun in that?? It makes it too easy! You might as well not play at all!
Freddo8 years ago
8 years ago
8 years ago
Hi, I don't have any under the hand right now, but expect a follow up with an example in the following week.
8 years ago
I use dots too... I don't want to get into a really long explanation, but the theory is basically the same as writing each number in every empty box except that the position of the dot represents a number (like on a telephone.)
8 years ago
Hm.. sounds interesting. I'm not totally understanding your method though. What do you mean by "Scanning each number once per 'quadrant', it helps to know which numbers come often and which one to try to complete first. This will let me discard more of them later." and "I start adding dots to the intersections of the lines and columns containing the more different numbers." I wish I could solve a sudoku in less than five minutes... please email me sloria1@gmail.com
I liked the suggestions. I have my own way of solving sudoks but it may take some time for me to solve. Its a dynamic logic processing technique (if I may use programming language). Easy sudoks take about 3-5 mins. The medium types take 7-8 mins. The difficult takes about 12 mins. And the very difficult take about 20-30 mins fo me. How much time will the above take?
cvillewill8 years ago
I did this exact method without writing down all of the little numbers. Since trying that my IQ has lowered 30 points. I am the first person with a negative IQ. At first I celebrated that and then I realized I lost my IQ to a dumb Japanese puzzle. I tried your method and I once again have a positive I.Q. ... Barely
domain manager8 years ago
free Sudoku Puzzles resource: http://www.glass.in/result.php?Keywords=free+sudoku
got lots of information.
popshed8 years ago
When I was wasting man hours doing Sudoku for a couple of weeks last year, I came up with a similar approach. It reached the point where every "puzzle" became a number crunching exercise, and not a particularly fun one at that. Once it had been reduced to this purely logical formula it became pointless and my two week addiction was destroyed. The only ones this can't solve are the really difficult ones which, despite what anyone says, all boil down to Ariadne's Thread solutions, again just number crunching until you arrive at the solution. Pointless. Well done for letting everyone know.
mightywombat8 years ago
Okay, but so why even bother to do SuDoKu now, if it's not a puzzle anymore?
Ravi रवि8 years ago
This no thinking Sudoku Solution is really Great!
N88 years ago
I just "see" the answers come now. Brain Age game really helped me develop this ability. I love the option to "see wrong answers" immediately. It makes you only go for the unambigous choices for a final answer, which improves your ability.
Rubik's Cube = 25 seconds :)
zinged8 years ago
How about just not do a Sudoku at all?
Mister Sparkle8 years ago
Isnt't this just bypassing the fun of the game? But then again, I also purchased the books on how to beat the Rubicks cube and a book on memorizing Pac Man patterns.
spinach_dip8 years ago
Sudoku is about as much fun as watching paint dry. You put down all the possible numbers in a blank square after eliminating all candidates that appear in the row, column or quadrant. Then you work through, using permutation and analysis, the squares that have multiple candidates. It being easy for a computer to solve just about says it all. The only interesting problems are ones that a computer finds hard to solve.
8 years ago
Word! And finding the time to write reponses to an online Sudoku blog, more fun than sex in space!
8 years ago
Sorry, I didn't mean to rain on your parade. But this AMAZING! SUDOKU! DISCOVERY!!! is clogging up my RSS feeds as the tidal wave approaches the shore.
jan.vdbergh8 years ago
In case you're interested in this technique: we have a sudoku game on our website that allows exactly this.

Check it out at http://www.sokudo.be/sudoku
joelfinkle8 years ago
I forgot to mention: check out http://www.ironsudoku.com for a free daily puzzle with nice ajax-y user interface for marking numbers.
8 years ago
Is there a method where you do not have to write all those numbers? I rarely have a pencil with a good enough eraser .

Thanks for the tips though.
One of the tricks that I seem to be employing (quite frequently) is the one you describe.
There is another one where if a number (say 4) has multiple choices within one box and either one row (or column) then you can eliminate that the other boxes in that row (or column).

I like the interface at http://www.zenofsudoku.com. You do not need to be connected :-).. It also has different levels so you can "train". The master level needs a black belt.
fstopblues8 years ago
A while ago, I made a small application that solved Sudoku for me. I was at work and very bored. I used the exact algorithm described here, with one improvement.
When all else fails with the steps above, you still have one more option. I call it "rail" elimination. If, for instance you have a "quadrants" where two (or even three) spaces are empty, but all in the same row:
3 _ 6
2 _ 8
5 _ 1
Then you can deduce that all other spaces in the row in the other quadrants will NOT have those numbers still missing from the first quadrant. This can help in situations like this:
1 2 3 _ _ 6
_ _ 6 7 _ 9 <== rail for 4 & 5
7 8 9 1 2 3
You know that 4 or 5 then can't be in the middle of the second quadrant, and so it must be 8. This sounds intuitive, but my app had to explicitly be told to do this.

HOWEVER, evem with this addition, the app still can't solve level 5 sudoku without educated guessing.

Here's the link to the app on my website:
www.getpoorquickscheme.com/Sudoku.exe
(I can put source code up too if someone wants to see it)
8 years ago
Solving Sudoku with a computer is trivial - you want to use a backtracking type scheme. The pseudo-code is like this: solve(first square) function solve(location) if location is end of puzzle: return "yes" if location has no possible choice: return "no" repeat for each possible choice write number in blank if solve(next location) then return "yes" end if end repeat erase number in this grid location return "no" end function I've implemented the above - it should take just a few minutes to write - and it works fine.
jmzero8 years ago
Here's a much, much simpler solution, and it's also foolproof. Once you've done the first 4 steps, you never have to do them again!

Prep work:
1. Write down all the numbers up to 1064.
2. Cross out any that contain zeroes.
3. Transfer each number into a blank Sudoku puzzle. Cross out any number that doesn't constitute a valid solution.
4. You now have the master list of Sudoku puzzles.

You may want to make a couple photocopies of this so that you can use one list for each puzzle you do. Make sure the copier has at least 1059 or so sheets of paper when you start - you don't want to run out half way through the job! Or you can just use a pencil when crossing out the numbers.

Solving a puzzle:
1. Cross out any numbers off the list that don't match the puzzle you're looking at.
2. At the end, you should be left with one number - that's the solution!

If there's more than one number, either you screwed up or whoever made the puzzle did. You should do all the steps again to make sure.
emaurin8 years ago
when a number is repeated like in step 9 (4-8 - 4-8) in a block or one column or row, they can discard it of the rest of the cells
Ok - I'm a SuDoku freak so I tried your method. My own method accomplishes the same thing and doesn't involve having to write so much. (I play a digital copy.) Each number will appear one time in each "quadrant." there are 3 quadrants across and down, of course... so each one will contain 1 - 9 in the middle, upper, or lower row. (left, middle or right for columns) Once you fill in as many as you can that way, then you can start filling in columns and rows. I solved the above "easy" puzzle pretty quickly using my own method - no erasing. When I tried this method (I used excel - too much penciling!), on a harder puzzle, there were way too many fields still unfilled. As mentioned above, a more analytical thinking process has to be applied even after eliminating as many as you can using whatever method. I'll read up some more on these other links to see if there are new ways of thinking that I can apply to my obsession.... ladyjake
LewisDowney8 years ago
Let me add a simple observation. The notepad I linked to above is simply that. The user has to determine by logic which numbers are or are not not possible. As I worked through ever more difficult Sudokus I could not keep up with all different combinations I was thinking through. So I would start little notes on a piece of paper. Later, I decided to organize the notes resulting in http://www.cicada.com/pdf/sudoku.pdf

The use of the notepad may well be outside of the spirit of the Sudoku, I cannot say. I can say that the act of keeping notes will not solve the puzzles by itself - the user must work through the necessary logic and intricasies to find the solution.

So my note pad is really simply a piece of paper to make notes on -- that because my memory could not keep up with the combinations of numbers I was trying to keep straight. Let me reiterate that the act of making notes will not remove the burden of solving the puzzle. It is easy to make mistakes and then mark them down. Those mistakes then become the foundation of solutions that do not work. To those of you who can resolve the puzzles without making notes, my kudos!
jridley8 years ago
Uh, this is totally obvious, I'm not sure how you'd solve sudokus except for doing this. At least, trivial ones. This is what a kid would figure out in 10 minutes. Note that this only works against "easy" and MAYBE some "medium" sudokus. Harder sudokus are going to require much more in-depth thinking, including looking ahead N moves; to solve on a computer you need tree or recursion algorithms.
logicchop8 years ago
This technique is misleading. It is not a "Sudoku algorithm," contrary to the way it is being portrayed. An algorithm would solve any arbitrary Sudoku puzzle, but this technique does not do that. Instead, it only solves relatively simple puzzles. In fact, if you look at the puzzles that this technique can solve, it would probably be easier to use a more straightforward technique (think of how much time you spend penciling in numbers). If you don't believe me, try solving an 'evil' puzzle on websudoku.com using this technique. You will quickly see that it doesn't get you very far. One more thing. Mark Frauenfelder posted a link to this page today on BoingBoing.net; he also criticizes Sudoku by contrasting it with crossword puzzles, which, as he puts it, are "clever, surprising, and educational." He also happens to think that this technique of penciling-and-eliminating is a "fullproof Sudoku solving algorithm." But why think that? Did someone post a proof somewhere? I didn't see one. Perhaps Mr. Frauenfelder should check the educational worth of his crossword puzzles because apparently he's not picking up basic reasoning skills. \Sudoku fan
ShedLab8 years ago
You don't happen to own a copy of Brain Age for the Nintendo DS do you? This is the exact same method you can use on that program as it allows you to write 9 small numbers in each square...
Bunky Strutts8 years ago
P.S. Do it with a pencil without an eraser, or better yet, in ink.
Bunky Strutts8 years ago
Folks, All you need to have to solve Soduku Puzzles quickly, is an audience. I've solved a few, but it does not give me the same satisfaction of cranking out a crossword. Instead, take the section of the daily paper to the soccer game, airport queue line, or anywhere someone might notice that you are working on a Sudoku grid. Fill numbers into the grid at random, stopping occasionally to scratch your head or take a sip of something. Finish filling in the grid quickly, lean back, and exhale. Then tear out the grid from the paper, fold it, and put it in your pocket. If anyone was looking over your shoulder, or otherwise checking you out, exclaim that you just broke your own record, and ask them to check it. Then pull out the puzzle grid from YESTERDAY'S PAPER from the same pocket. Of course you have to fill in the published solution from TODAY'S PAPER to pull this off. You can only do this every other day, unless you purchase two daily newspapers. Bunky Strutts
acidzebra8 years ago
I'm with Marty here; this will only work on the simplest of Sudoku.
martymorrow8 years ago
any puzzle that can be solved this way would be considered "easy" Yes, doing easy puzzles is boring. So is solving easy crosswords. Try a puzzle with less than 25 numbers displayed. Yours has 30.
ericgorr8 years ago
While this method will solve the simplest of the Sudoku puzzles, it is wholly inadequate to solve anything resembling an interesting Sudoku puzzle.

For example, one will not be able to solve this puzzle:

http://www.scanraid.com/sudoku.htm?bd=620401030154008076003600401002547169475100382900823040008004600040900010000700054

Using the method described above.

The Sudoku solves this link will take you to cannot even solve it and yet there is a logically determined move one can make to solve the puzzle. And, no, I won't tell you what it is.

For many excellent strategies one might have to employ to solve a Sudoku puzzle, you can check here:

http://www.scanraid.com/BasicStrategies.htm#NP

and other sites found via google.

Basically, this person who wrote:

"It's so easy, you don't even have to think!"

doesn't even have a remote clue what they are talking about.

8 years ago
It's true this isn't a complete solution guide, but doesn't linking to scanraid fail to change the issue at hand here? Once one learns how to apply searching for naked triples, do all the various x-wings, and so on, isn't the solution just as mechanical? The sudoku solver on the page that you link to is just a version of the above, just with a wider array of strategies.

Sure, if you want to solve sudokus fast, you need to become good at recognizing which strategies are more likely to work, which is some form of pattern-matching/intuition, but for the simple act of solving, all you need is a sufficiently powerful set of strategies and the willingness to spend a bit of time.*

• I concede that there may be sudoku's so powerful that one would need to invent new strategies to solve them, but hopefully we can agree that this is a very limited and extremely difficult to reproduce to set.
LewisDowney8 years ago
Last year I created a little pdf note pad to support an approach very similar to the idea discussed here. It has an enlarged grid with all the possible numbers in it (1 thru 9 for each cell). For the row and columns there is an exterior grid of numbers. Instead of erasing, I simply strike through the numbers that are not possible - eventually arriving at the correct combinations in all directions. The winnng number, so-to-speak, for each cell is printed in the space available underneath the 1-9 strikeout numbers. The seed numbers are also put in that space of course.

If these instructions do not make sense, leave a note here over the next couple of days; and I'll see if I can fill in any gaps.
dechunter8 years ago
When I first saw sudoku, the technique described here seemed to me to be the intuitive method for solving the puzzles. How else do people do it?
julesa8 years ago
No, you never have to guess. You just have to use other tricks. Say you have 469 2346 49 2369 469 in a row, notice the 469, 49, 469. Those three squares have used up those three numbers, so you can eliminate 4, 6, and 9 from the other squares in that row, leaving 469 23 49 23 469.
joelfinkle8 years ago
There are a few techniques which can help for harder puzzles:
Examine each of the marked cells in a (row, column, box): if there are two numbers that only appear in two of the boxes, then you know that those two boxes can only have those two numbers.

Example: If one row has 4567, 4589, 6789, and 6789 left, then the leftmost can only be 45, and the rightmost, by process of elimination, can only be 89.

More rarely, you can find the same rule works for 3-way and 4-way groups.

Another trick: When scanning a row or column, and you find that a number only appears in one box within that row, you can eliminate that number from all other cells in that box. Conversely, when scanning a box, if a number appears in just one column (or row), you can eliminate that number from the other boxes that that column or row passes through.

At the hardest levels, you will find you DO have to guess between sets of possible results. A badly-created puzzle will have multiple solutions (tsk tsk), but most of the time you will need to test an assumption *carefully* such as circling the ones you *think* might be right, and putting tick-marks above the numbers that the circled numbers then rule out. Don't erase anything will testing assumptions, because it's very hard to backtrack.
delgaudm (author) 8 years ago
Freddo, I think you are right, as far as this system won't solve the most difficult puzzles. If there is a puzzle with so few clues that you have to make a "guess" as to the next move, or think ahead then the pure "process of elimination" won't work. I like your system with the dots. I had tried something similar by using entering in the numbers as the were eliminated in the grid -- in fact, that was the first way I tried it. For me, it was confusing, my personal preference was to eliminate, as it made recognizing the numbers to eliminate easier. Thanks for the great feedback!
pattern868 years ago
word.
azn1stknightsoul8 years ago
Thats pretty good. I think that with this strategy, I may even beat my teacher who is a sudoku freak. Great job man.
ag100man8 years ago
I like it. Reduces Sudoku to a straightforward procedure and very nicely explained.