返回 list.append() 在递归函数中抛出"TypeError: unsupported operand type(s)"

Question

我尝试编写一个递归函数来返回最长的递增子序列，但发生错误说" TypeError: unsupported operand type(s) for +: 'NoneType' and 'NoneType' ">

def helper(cur_seq, seq, cur_i, result):
    if len(seq) == cur_i:
        return result.append(cur_seq)
    else:
        next_i = cur_i + 1
        if len(cur_seq) == 0 or seq[cur_i] > cur_seq[-1]:
            temp = cur_seq.copy()
            temp1 = cur_seq.copy()
            temp.append(seq[cur_i])
            return helper(temp, seq, next_i, result) + helper(temp1, seq, next_i, result)
        else:
            return helper(cur_seq.copy(), seq, next_i, result)

def longest_sub_sequence(seq):
    cur_seq = []
    result = helper(cur_seq, seq, 0, [])
    max_length = 0
    for i in result:
        if len(i) > max_length:
            max_length = len(i)

    return max_length

if __name__ == "__main__":
    seq = [0, 8, 4, 12, 2, 10, 6, 14, 1, 9, 5, 13, 3, 11, 7, 15]
    y = longest_sub_sequence(seq)
    print(y)

Answer 1

list.append是一个就地操作。例如

l = [1, 2, 3]
result = l.append(4)
print(result)
# None
print(l)
# [1, 2, 3, 4]

append返回None，就地修改l。这意味着函数中的return result.append(cur_seq)将返回None，并且两个递归调用会导致None + None，从而为您提供TypeError。

解决方法是先附加，稍后返回。

def helper(cur_seq, seq, cur_i, result):
    if len(seq) == cur_i:
        result.append(cur_seq)
        return result
    ...