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

class Solution:
    def diameterOfBinaryTree(self, root: TreeNode) -> int:
        self.maxlen = 1
        def deep(root):
            if not root:
                return 0
            L = deep(root.left)
            R = deep(root.right)
            self.maxlen = max(self.maxlen, L + R + 1)
            return max(L, R) + 1

        deep(root)
        return self.maxlen - 1