37. 解数独

主要是中间那段调用 if self.backtracking(board): 返回True，以及后面一个return False和return True不好理解。之后还要深入理解

class Solution:
    def solveSudoku(self, board):
        self.backtracking(board)

    def backtracking(self, board):
        for i in range(len(board)):  # 行
            for j in range(len(board[0])):  # 列
                if board[i][j] != ".":
                    continue
                for k in range(1, 10):
                    if self.is_valid(i, j, k, board):
                        board[i][j] = str(k)
                        if self.backtracking(board):
                            return True
                        board[i][j] = "."
                return False
        return True

    def is_valid(self, i, j, k, board):
        for l in range(9):
            if board[i][l] == str(k):
                return False

        for m in range(9):
            if board[m][j] == str(k):
                return False

        row = i // 3
        col = j // 3
        for s in range(row * 3, row * 3 + 3):
            for t in range(col * 3, col * 3 + 3):
                if board[s][t] == str(k):
                    return False

        return True


board = [["5", "3", ".", ".", "7", ".", ".", ".", "."],
         ["6", ".", ".", "1", "9", "5", ".", ".", "."],
         [".", "9", "8", ".", ".", ".", ".", "6", "."],
         ["8", ".", ".", ".", "6", ".", ".", ".", "3"],
         ["4", ".", ".", "8", ".", "3", ".", ".", "1"],
         ["7", ".", ".", ".", "2", ".", ".", ".", "6"],
         [".", "6", ".", ".", ".", ".", "2", "8", "."],
         [".", ".", ".", "4", "1", "9", ".", ".", "5"],
         [".", ".", ".", ".", "8", ".", ".", "7", "9"]]
Solution().solveSudoku(board)
for i in board:
    print(i)