https://leetcode.cn/problems/diameter-of-binary-tree/ (opens new window) 需要注意的点

1. 左右子树的和是该节点的直径

# 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:
    maxDiameter = 0
    def diameterOfBinaryTree(self, root: Optional[TreeNode]) -> int:
        if not root:
            return 0
        

        def depth(node: Optional[TreeNode]) -> int:

            if  not node:
                return 0

            left = depth(node.left)
            right = depth(node.right)
            # 左右子树深度和
            self.maxDiameter = max(self.maxDiameter, left+right)
            return max(left,right)+1

        depth(root)
        return  self.maxDiameter