有效的数独

因为每次按行遍历，因此建立一个行哈希表，9个列哈希表，和3个方格哈希表，通过查询元素是否在相应表中来判断：

class Solution:
    def isValidSudoku(self, board: List[List[str]]) -> bool:

        columnsValid = [{}, {}, {}, {}, {}, {}, {}, {}, {}]
        for i, r in enumerate(board):
            rowsValid = {}
            if i % 3 == 0:
                gridsValid = [{}, {}, {}]
            for j, c in enumerate(r):
                if c != '.':
                    if c in rowsValid:
                        return False
                    else:
                        rowsValid[c] = True
                    if c in columnsValid[j]:
                        return False
                    else:
                        columnsValid[j][c] = True
                    if c in gridsValid[j // 3]:
                        return False
                    else:
                        gridsValid[j // 3][c] = True     

        return True