```
# 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):
def diameterOfBinaryTree(self, root):
"""
:type root: TreeNode
:rtype: int
"""
def diameterOfBinaryTreeIter(root):
if not root:
return 0, 0
leftlen, leftmax = diameterOfBinaryTreeIter(root.left)
rightlen, rightmax = diameterOfBinaryTreeIter(root.right)
return 1 + max(leftlen, rightlen), max(leftmax, rightmax, leftlen + rightlen)
return diameterOfBinaryTreeIter(root)[1]
```