判断一个 9x9 的数独是否有效。只需要根据以下规则，验证已经填入的数字是否有效即可。
    数字 1-9 在每一行只能出现一次。
    数字 1-9 在每一列只能出现一次。
    数字 1-9 在每一个以粗实线分隔的 3x3 宫内只能出现一次。
    链接：https://leetcode-cn.com/problems/valid-sudoku

    1. 输入:
    2. [
    3.   ["5","3",".",".","7",".",".",".","."],
    4.   ["6",".",".","1","9","5",".",".","."],
    5.   [".","9","8",".",".",".",".","6","."],
    6.   ["8",".",".",".","6",".",".",".","3"],
    7.   ["4",".",".","8",".","3",".",".","1"],
    8.   ["7",".",".",".","2",".",".",".","6"],
    9.   [".","6",".",".",".",".","2","8","."],
    10.   [".",".",".","4","1","9",".",".","5"],
    11.   [".",".",".",".","8",".",".","7","9"]
    12. ]
    13. 输出: true
    1. class Solution {
    2. public:
    3.     bool isValidSudoku(vector<vector<char>>& board) {
    4.         vector<unordered_map<int, int>> rows(9), cols(9), blocks(9);
    5.         for (int i = 0; i < 9; i++) {
    6.             for (int j = 0; j < 9; j++) {
    7.                 if (board[i][j] == '.') continue;
    8.                 int blo_index = (i/3)*3 + (j/3);
    9.                 int cur = board[i][j] - '0'; //字符转成Int
    10.                 if (rows[i].count(cur) || cols[j].count(cur) || blocks[blo_index].count(cur)) 
    11.                     return false;
    13.                 rows[i][cur] = 1; //行记录一次 
    14.                 cols[j][cur] = 1; //列记录一次 
    15.                 blocks[blo_index][cur] = 1; //块记录一次   
    16.             }
    17.         }
    18.         return true;
    19.     }
    20. };
    22. //可简化成二维数组的形式，用二维数组记录状态。