Solve Sudoku (Without Even Thinking!)




Introduction: 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.

Step 1: What You Need to Begin

You need:

1. A sudoku puzzle. I enlarged this one (which was the puzzle that appeared on 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.

Step 6: Repeat for All the Pre-printed Numbers

Just repeat thos three steps for each number that was printed in the puzzle. The purpose of the down, across, and circle marks on each number is to provide for any interruption that may occur when solving the puzzle. By drawling those lines you can always tell exactly where you left off. If the number has all three marks, you can be sure that all of the corresponding numbers have been eliminated from the puzzle. YOu can effectively forget about that block now.

Let's see what the result of all that erasing was.

Step 7: Locate Answers

By process of elimination you will have done so much erasing in some blocks that only one number is left. Guess what. THat is the answer for that block. Congratulate yourself.

Write that number large in the block, then treat it like a preprinted number. Eliminate all the across, down and around numbers as you have already done so many times before.

In this image, the red pen points point to all the answers that were provided just by doing the initial erasing.

We have not yet begun to think!

Step 8: Other Kinds of Eliminations.

You won't always be so lucky. Not every time will you end up erasing all the numbers but one in a block. If that is the case, then you peruse the puzzle to find eliminations.

First look down each column. If you find a number that only appears in one block in a column. That is the answer for that block.

In this picture, the pen point points to a three. That is the only three in that column, so that must be where it goes.

Write it large, then treat it like a pre-printed number. See ... this isn't thinking. This is EASY.

Step 9: Look Around a Quadrant.

I know, they aren't really quadrants, but you know what I mean, don't you?

If you've looked across the columns, and across the rows and there are all duplicates, then look in the group. If there is one number in the group that only appears in one block, then it's your answer.

See... There is no guessing in Sudoku. One of these situations has to occur. You just need to find them.

IN this picture the 5 only appears in one block in the group. It goes there.

Write it big, then treat it like a pre-printed number.

See where we're going, here?

Step 10: Lather, Rinse Repeat.

Eventually, you will find all those lonely numbers in thier rows, columns or groups.

Remember to draw those lines and circles in case you get interrupted.

Eventually you will solve the puzzle.

Compare our solution to the "Official Solution" found on the site. See? A perfect match.

And you thought Sudoku was hard.

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

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265 Discussions

What if each number repeats more than once


7 months ago

I did try it, so now I have to take back what I said about still needing to guess. I tried the hardest puzzles I could find, and the worst trouble I had was having to return to classic solving methods a few times. At one point I made a mistake. Don't know if it was because of the option mapping, but had no trouble getting back on track.

This is still tedious, but that can be minimized by solving as much with traditional methods as you can, and only then start mapping solutions. I do want to point out that sudoku programs have tools for this very aid, and that it is considered an aid, not cheating. It's like setting the program to highlight a given number. Helps you see what to do easier, that's all.


7 months ago

First, the word you're looking for is sector.

Second, all this writing and erasing is time consuming, but will get you to the end of most puzzles. It's a good way of using what every newbie tries at first, only to end up with a screaming headache. But you made great sense of it. Nicely done.

Third, as anyone who has been through an entire sudoku puzzle book can tell you, there is guessing in sudoku, and it definitely helps to have rules for that once you get to the really hard puzzles.

So at some point you will want to leave this method behind, grab a pen and pencil, and get to guessing, because the really hard ones give you no choice but to guess. Using a pen makes it easier not to miss the entries you make (when I used a pencil, I kept missing mine and it drove me crazy), and when you need to guess you can reduce each number to a small, unique section of the number. 8, for instance, could by a tiny x in the middle of the square. Make of note of where you start and the number you wrote on the puzzle's edge. If you make it small, you won't make a mess of the puzzle by being wrong. And once you find you are wrong, you can enter the alternatives you now know are correct.

Now that you have wrong marks, using the pen to guess will only confuse you. Guess with the pencil, and bear in mind that you can guess as much as three numbers at a time under the right conditions, and it will still be an either or or option.

Took ages but I can confirm the method works for a hard sudoku. Very logical way of solving a sudoku, but probably not the most efficient way.

This method is very thorough and if you follow it exactly, you will be able to solve an easy sudoku puzzle. However, it requires a lot of time to write down every possibility in every square. I think it might be overkill for an easy puzzle. Most easy sudokus can be solved just by looking. Here's a video that shows you how to solve easy sudoku puzzles.

this is genral logic, but i want formulae for solve easily.......!!!

1 reply

Certainly a decent way to solve simple puzzles, but make no mistake - solving one sudoku puzzle does not mean solving them all. The above Sudoku puzzle is classified as easy; I can finish it in under a minute without even writing down possibilities.

This basic elimination method is generally the last thing you would be doing for intermediate and advanced puzzles. The hardest Sudoku puzzles require a good amount of logical thinking, observation, and occasionally memory.
I'd be more than happy to create an advanced puzzle for anyone interested to see if the above method can even fill up a single column, row, or box.


I'm new to Sudoku. The tip about the circles, vertical and horizontal bars is a great help. Thanks!

Why don't you make some amazing puzzle game by yourself easily within days? and make some quick money from it.:

I solved it in 6 minutes.

I appreciate all of your work, but this makes the task way more complicated now.
Plus, this isn't a tip for playing. Its just another kind of cheating, right?



This is brilliant strategy, but won't it take like forever to finish ?


2 years ago

Nice but too slow - with thinking is much faster...

btw thanks 4 share