Sudoku Solver

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

Empty cells are indicated by the character '.'.

You may assume that there will be only one unique solution.

A sudoku puzzle...

...and its solution numbers marked in red.

https://leetcode.com/problems/sudoku-solver/

Solution :

https://leetcode.com/discuss/9929/a-java-solution-with-notes

public class Solution {
    public void solveSudoku(char[][] board) {
        if (board.length == 0) {
            return;
        }
        solve(board);
    }

    public boolean solve(char[][] board) {
        int m = board.length;

        for (int i = 0; i < m; i++) {
            for (int j = 0; j < m; j++) {
                if (board[i][j] == '.') {
                    for (char c = '1'; c <= '9'; c++) {
                        if (isValid(board, i, j, c)) {
                            board[i][j] = c;
                            if (solve(board)) {
                                return true;
                            } else {
                            board[i][j] = '.';
                            }
                        }
                    }
                    return false;
                }
            }
        }
        return true;
    }

    public boolean isValid(char[][] board, int i, int j, char c) {
        // check col
        for (int row = 0; row < board.length; row++) {
            if (board[row][j] == c) {
                return false;
            }
        }
        // check row
        for (int col = 0; col < board.length; col++) {
            if (board[i][col] == c) {
                return false;
            }
        }
        // check block

        for (int row = (i/3)*3; row < (i/3)*3+3; row++) {
            for (int col = (j/3)*3; col < (j/3)*3+3; col++) {
                if (board[row][col] == c) {
                    return false;
                }
            }
        }

        return true;
    }
}