687.py

# 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 longestUnivaluePath(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        longest = [0]
        def traverse(node):
            if not node:
                return 0
            left_len, right_len = traverse(node.left), traverse(node.right)
            left = (left_len + 1) if node.left and node.left.val == node.val else 0
            right = (right_len + 1) if node.right and node.right.val == node.val else 0
            longest[0] = max(longest[0], left + right)
            return max(left, right)
        traverse(root)
        return longest[0]