求助】判断语句无效，加了flag也还是不行。

代码如下：
def solve(sudoku):
    flag = 0
    list_a = reduce_vr(sudoku)
    list_temp = []
    for i in range(9):
        for j in range(9):
            if len(list_a[i][j]) != 1:
                flag = 1
            else:
                break
    if flag == 1:
        list_temp = trial(sudoku)
        print("1")
    else:
        list_temp = list_a
        print("2")

        return list_temp

run结果如下：
please input your sudoku question:000502000016000080000000000070030010060000000500009000030080000000000200700000509
2
[[8], [9], [4, 7], [5], [1, 4, 6, 7], [2], [1, 3, 4, 6, 7], [3, 4, 6, 7], [1, 4, 7]]
[[2], [1], [6], [3, 4, 7], [9], [3, 4, 7], [3, 4, 7], [8], [5]]
[[3], [5], [4, 7], [1, 4, 6, 7], [1, 4, 6, 7], [8], [1, 4, 6, 7], [9], [2]]
[[4], [7], [2], [8], [3], [5], [9], [1], [6]]
[[1], [6], [9], [4, 7], [2], [4, 7], [8], [5], [3]]
[[5], [8], [3], [1, 4, 6, 7], [1, 4, 6, 7], [9], [4, 7], [2], [4, 7]]
[[9], [3], [5], [2], [8], [1, 4, 6, 7], [1, 4, 6, 7], [4, 6, 7], [1, 4, 7]]
[[6], [4], [1], [9], [5], [3, 7], [2], [3, 7], [8]]
[[7], [2], [8], [1, 3, 4, 6], [1, 4, 6], [1, 3, 4, 6], [5], [3, 4, 6], [9]]

Process finished with exit code 0