二叉树递归方法的直径

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

class Solution:
    def diameterOfBinaryTree(self, root: TreeNode) -> int:
        self.diameter = 0
        
        def depth(node):
            if not node:
                return 0
            left_depth = depth(node.left)
            right_depth = depth(node.right)
            self.diameter = max(self.diameter, left_depth + right_depth)
            return max(left_depth, right_depth) + 1
        
        depth(root)
        return self.diameter