Sudoku Solver

Question 1

What is a Sudoku? (Definition)

Answer

Sudoku is a puzzle game whose goal is to fill a grid with the numbers from 1 to 9 (there are variants with letters or symbols) so that they appear only once per row, column or block.

Question 2

How to fill the Sudoku grid?

Answer

Make a copy/paste or type directly in cells. Letters (Wordoku/alphadoku) and digits are accepted.

Example: Copy the Sudoku content in the first cell (top left) 97 1 5 5 9 2 18 4 8 7 26 92 3 6 2 9 19 4572 to get:

9	7	␣	1	␣	␣	␣	␣	5
␣	␣	5	␣	9	␣	2	␣	1
8	␣	␣	␣	4	␣	␣	␣	␣
␣	␣	␣	␣	8	␣	␣	␣	␣
␣	␣	␣	7	␣	␣	␣	␣	␣
␣	␣	␣	␣	2	6	␣	␣	9
2	␣	␣	3	␣	␣	␣	␣	6
␣	␣	␣	2	␣	␣	9	␣	␣
␣	␣	1	9	␣	4	5	7	2

Question 3

How does the step by step works?

Answer

The program solves the 9x9 sudoku much like a human would, calculating the possible values in each box and making deductions.

— Only possible value indicates that, in this box, any other value (1-9) results in an error/inconsistency (in a row, column or region).

— Only box possible indicates that all other boxes (within a block/region) cannot contain the value.

For each validated box/number, the solver displays an additional step in its progress in order to understand.

Sometimes the reasoning arrives at a stage where no quick logical deduction allows to deduce the value of a box (several possible values). The solver then analyzes the boxes where there are the fewest possibilities and selects a value that it considers the most probable and continues, if an inconsistency occurs (that the sudoku has no more solution) then it returns to step selection and takes another value. To optimize the chances, the selection is not random, the choice is made on a number which, if it is positioned in the box, will allow the maximum deduction for the next sudoku steps. This method is the fastest for the solver, it may be that sometimes a slower and complex deduction could achieve the same result.

Question 4

What is a block of Sudoku?

Answer

In a sudoku, the 9x9 grid presents 9 squares of 3x3 which are called blocks (or regions, or groups, or sectors or sub-grids).

Question 5

How to test if a sudoku has a unique solution?

Answer

dCode calculates all the solutions for the sudoku, not only the first one. To check a homemade sudoku, the solver can confirm that there is only one solution.

Question 6

When was Sudoku invented?

Answer

The first Sudoku versions are from 1979

Question 7

How many distinct sudokus exist?

Answer

A single row can consist of $ 9! $ (Factorial of 9) different ways, same for the columns, but the whole sudoku has a number of possibilities much less than $ 9!^9 $, because some permutations can lead to identical grids. The total number of grids would be $ 9! \times 72^2 \times 2^7 \times 27704267971 = 6670903752021072936960 $ combinations.

Question 8

What is the minimum number of starting squares for a unique solution?

Answer

For a sudoku to have only one solution, it must have at least 17 pre-filled boxes out of the 81 total. Gary McGuire and his team proved this here by using an algorithm to perform a systematic computer search. They first cataloged the 5,472,730,530 distinct sudoku puzzles. Next, they created a program to check whether sudoku puzzles with 16 pre-filled squares had a unique solution. Their exhaustive search found no sudoku with only 16 pre-filled squares having a unique solution, thus confirming the minimum number of 17 pre-filled squares needed.

Question 9

Does a rectangular sudoku or non square sudoku exist?

Answer

By keeping the sudoku NxN rules that require the N characters to be used on each row and column, then it is impossible to respect them if the sudoku is not square.

Some variants of the square sudoku, however, use non-square inner blocks (see sudoku 6x6, sudoku 7x7 or sudoku 8x8)

Sudoku Solver

Solve a sudoku grid

Answers to Questions (FAQ)

What is a Sudoku? (Definition)

How to fill the Sudoku grid?

How does the step by step works?

What is a block of Sudoku?

How to test if a sudoku has a unique solution?

When was Sudoku invented?

How many distinct sudokus exist?

What is the minimum number of starting squares for a unique solution?

Does a rectangular sudoku or non square sudoku exist?

Source code

Cite dCode

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Questions / Comments

9	7	␣	1	␣	␣	␣	␣	5
␣	␣	5	␣	9	␣	2	␣	1
8	␣	␣	␣	4	␣	␣	␣	␣
␣	␣	␣	␣	8	␣	␣	␣	␣
␣	␣	␣	7	␣	␣	␣	␣	␣
␣	␣	␣	␣	2	6	␣	␣	9
2	␣	␣	3	␣	␣	␣	␣	6
␣	␣	␣	2	␣	␣	9	␣	␣
␣	␣	1	9	␣	4	5	7	2

9	7	␣	1	␣	␣	␣	␣	5
␣	␣	5	␣	9	␣	2	␣	1
8	␣	␣	␣	4	␣	␣	␣	␣
␣	␣	␣	␣	8	␣	␣	␣	␣
␣	␣	␣	7	␣	␣	␣	␣	␣
␣	␣	␣	␣	2	6	␣	␣	9
2	␣	␣	3	␣	␣	␣	␣	6
␣	␣	␣	2	␣	␣	9	␣	␣
␣	␣	1	9	␣	4	5	7	2

9	7	␣	1	␣	␣	␣	␣	5
␣	␣	5	␣	9	␣	2	␣	1
8	␣	␣	␣	4	␣	␣	␣	␣
␣	␣	␣	␣	8	␣	␣	␣	␣
␣	␣	␣	7	␣	␣	␣	␣	␣
␣	␣	␣	␣	2	6	␣	␣	9
2	␣	␣	3	␣	␣	␣	␣	6
␣	␣	␣	2	␣	␣	9	␣	␣
␣	␣	1	9	␣	4	5	7	2