leetcode 37. 解数独

用深搜dfs，返回值boolean表示找到了。

class Solution {
    public void solveSudoku(char[][] board) {
        dfs(board);
    }
    boolean dfs(char[][] board){
        for(int i=0;i<9;i++){
            for(int j=0;j<9;j++){
                char ch=board[i][j];
                if(ch!='.') continue;
                for(char k='1';k<='9';k++){
                    if(isValid(board,i,j,k)){
                        board[i][j]=k;
                        if(dfs(board)) return true;
                        board[i][j]='.';
                    }
                }
                return false;
            }
        }
        return true;
    }
    boolean isValid(char[][] board,int i,int j,char num){
        for(int col=0;col<9;col++){
            char ch=board[i][col];
            if(ch==num) return false;
        }
        for(int row=0;row<9;row++){
            char ch=board[row][j];
            if(ch==num) return false;
        }
        int m=i/3,n=j/3;
        for(int r=m*3;r