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.

Solution: 
类似N-Queens问题， 一个个尝试填数，不行就回溯，直到填满返回。

如果对一个格子尝试从0~9都不行，那么说明整个sudoku无解，返回false就好。

对整个棋盘所有'.'都填完了，那么就可以返回true了。

public class Solution {
    public void solveSudoku(char[][] board) {
        if(board == null || board.length != 9|| board[0].length != 9) {
            return;
        }
        helper(board);
    }
    
    public boolean helper(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)) {  //try
                            board[i][j] = c;
                            if(helper(board)) {
                                return true;
                            }else {
                                board[i][j] = '.';  //go back
                            }
                        }
                    }
                    return false;
                }
            }
        }
        return true;
    }
    
    public boolean isValid(char[][] board, int i, int j, int c) {
        for(int m = 0; m < 9; m++) {  //check row
            if(m != j && board[i][m] == c) {
                return false;
            }
        }
        
        for(int m = 0; m < 9; m++) {  //check column
            if(m != i && board[m][j] == c) {
                return false;
            }
        }
        
        for(int row = i/3*3; row < i/3*3+3; row++) {  //check sub-boxes
            for(int col = j/3*3; col < j/3*3+3; col++) {  
                if((row != i || col != j) && board[row][col] == c)  
                    return false;  
            }  
        }
        return true;
    }
}