Problem Statement

Write a program to solve a Sudoku puzzle by filling the empty cells.

A sudoku solution must satisfy all of the following rules:

• Each of the digits 1-9 must occur exactly once in each row.
• Each of the digits 1-9 must occur exactly once in each column.
• Each of the digits 1-9 must occur exactly once in each of the 9 3x3 sub-boxes of the grid.

The '.' character indicates empty cells.

Example 1:
Input:

            {'5', '3', '.', '.', '7', '.', '.', '.', '.'},
            {'6', '.', '.', '1', '9', '5', '.', '.', '.'},
            {'.', '9', '8', '.', '.', '.', '.', '6', '.'},
            {'8', '.', '.', '.', '6', '.', '.', '.', '3'},
            {'4', '.', '.', '8', '.', '3', '.', '.', '1'},
            {'7', '.', '.', '.', '2', '.', '.', '.', '6'},
            {'.', '6', '.', '.', '.', '.', '2', '8', '.'},
            {'.', '.', '.', '4', '1', '9', '.', '.', '5'},
            {'.', '.', '.', '.', '8', '.', '.', '7', '9'}

Output:

            {'5'', '3'', '4'', '6'', '7'', '8'', '9'', '1'', '2''},
            {'6'', '7'', '2'', '1'', '9'', '5'', '3'', '4'', '8''},
            {'1'', '9'', '8'', '3'', '4'', '2'', '5'', '6'', '7''},
            {'8'', '5'', '9'', '7'', '6'', '1'', '4'', '2'', '3''},
            {'4'', '2'', '6'', '8'', '5'', '3'', '7'', '9'', '1''},
            {'7'', '1'', '3'', '9'', '2'', '4'', '8'', '5'', '6''},
            {'9'', '6'', '1'', '5'', '3'', '7'', '2'', '8'', '4''},
            {'2'', '8'', '7'', '4'', '1'', '9'', '6'', '3'', '5''},
            {'3'', '4'', '5'', '2'', '8'', '6'', '1'', '7'', '9''}

Explanation: The given output is the only valid Sudoku solution.

Constraints:

• board.length == 9
• board[i].length == 9
• board[i][j] is a digit or '.'
• It is guaranteed that the input board has only one solution.

Try it yourself

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