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.

思路；
DFS + BackTracking

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

    public boolean solve(char[][] board){
        for(int i=0; i<board.length; i++){
            for(int j=0; j<board[0].length; 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;
    }

    private boolean isValid(char[][] board, int row, int col, char c){
        for(int i = 0; i < 9; i++){
            if(board[row][i] == c || board[i][col] == c) return false;
            if(board[row / 3 * 3 + i / 3][col / 3 * 3 + i % 3] == c) return false;
        }
        return true;
    }
}