# Free Sudoko Solver

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## Four techniques to Solve Sudoku

Sudoku is a puzzle involving logic and no arithmetic or guessing is required.  The basic idea of completing puzzles is to find cells-the small squares, where it is sure that only one value is a valid placement. The basic rules of Sudoku are that fill a number in to every cell in the grid, using the numbers 1 to 9. The constraint is that use each number only once in each row, each column, and in each of the 3×3 boxes. Given below, four techniques can help you solve Sudoku puzzle more easily and quickly.

## Single Position

This is the simplest technique to apply by eye, select a row, column or box, and then go through each of the numbers that has not already been placed. Because of other placements, the positions where the number could be placed will be limited. Often there will be two or three places that are valid, sometimes, there will only be one place. If you have narrowed it down to only one valid place where you can put the number, you can fill that number straight in, since it cannot go anywhere else. Sometimes this technique is referred to as a ‘Hidden Single’ if the value is hidden alongside other possible candidates. You should still be able to spot it though.

## Single Candidate

If you are using pencil marks to store what candidates are still possible within each cell. If you’ve managed to rule out all other possibilities for a particular cell by examining the nearby column, row and box, so there is only one number left that could possibly fit there – fill in that number. Once you have filled these in, you will soon see lot of more single candidates, and with this technique you may be able to go on to complete many of the simpler Sudoku puzzles.

If you are using a computer program to assist you, then you will probably do most of your placements with this method.  If you are doing your pencil marks by hand, double check that you have filled them in otherwise you might make a placement that is not valid. Going a little further, there are some extra techniques which assist you to find either valid placements, or to assist you to remove some of the pencil marks. These are certainly quite tricky to manage without using pencil marks.

## Candidate Line

This technique does not actually tell you where to place a number, but instead assists you to find out places where you cannot place a number. If you are re using pencil marks, then this will assist you to remove candidates, and from there you should be able to make placements. If you look within a box, and find that all of the places where you can put a particular number lie along a single line, then you can be sure that wherever you put the number in that box, it has to be on the line.

## Double Pair

This technique relies on spotting two pairs of candidates for a value, and using these to rule out candidates from other boxes. This technique is logically easy to spot because you only need to see candidate pairs in two blocks.