Leetcode 1372：为什么这两个代码片段给出不同的结果？

Question

我正在解决leetcode问题1372。为什么这两个代码返回不同的结果？第一个给出了正确答案。第二个没有。

# Definition for a binary tree node.

# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def __init__(self):
        self.longest_path = 0

    def longestZigZag(self, root: Optional[TreeNode]) -> int:
        val1 = 1 + self.helper(root.right, 'r')
        val2 = 1 + self.helper(root.left, 'l')
        # print(val2, self.longest_path)
        val = max(val1, val2)
        return max(val, self.longest_path) - 1

    def helper(self, root, d):
        if root == None:
            return 0
        if d == 'l':
            l_path = 1 + self.helper(root.left, d='l')
            self.longest_path = max(self.longest_path, l_path)
            return 1 + self.helper(root.right, d='r')
        if d == 'r':
            r_path = 1 + self.helper(root.right, d='r')
            self.longest_path = max(self.longest_path, r_path)
            return 1 + self.helper(root.left, d='l')
        



class Solution:
    def __init__(self):
        self.longest_path = 0

    def longestZigZag(self, root: Optional[TreeNode]) -> int:
        val1 = 1 + self.helper(root.right, 'r')
        val2 = 1 + self.helper(root.left, 'l')
        # print(val2, self.longest_path)
        val = max(val1, val2)
        return max(val, self.longest_path) - 1

    def helper(self, root, d):
        if root == None:
            return 0
        if d == 'l':
            self.longest_path = max(self.longest_path, 1 + self.helper(root.left, d='l'))
            return 1 + self.helper(root.right, d='r')
        if d == 'r':
            r_path = 1 + self.helper(root.right, d='r')
            self.longest_path = max(self.longest_path, r_path)
            return 1 + self.helper(root.left, d='l')

唯一的变化是这一行：

“self.longest_path = max(self.longest_path, 1 + self.helper(root.left, d='l'))”

Answer 1

第二个代码丢失了

l_path = 1 + self.helper(root.left, d='l')

。