LeetCode 543 Diameter of Binary Tree

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    # https://leetcode.com/problems/diameter-of-binary-tree/solution/
    def diameterOfBinaryTree(self, root):
        self.ans = 1
        def depth(node):
            if not node: return 0
            L = depth(node.left)
            R = depth(node.right)
            self.ans = max(self.ans, L+R+1)
            return max(L, R) + 1

        depth(root)
        # number of nodes - 1 = length
        return self.ans - 1