LeetCode (Python): Binary Tree Maximum Path Sum

Given a binary tree, find the maximum path sum.

The path may start and end at any node in the tree.

For example:
Given the below binary tree,

       1
      / \
     2   3

Return 6.

Solution:

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

class Solution:
    # @param root, a tree node
    # @return an integer
    def maxPathSum(self, root):
        self.maxValue = float("-inf")
        self.maxPathSumRec(root)
        return self.maxValue
    
    def maxPathSumRec(self, root):
        if root == None:
             return 0
        leftSum = self.maxPathSumRec(root.left)
        rightSum = self.maxPathSumRec(root.right)
        if leftSum<0 and rightSum<0:
            self.maxValue = max(self.maxValue, root.val)
            return root.val
        if leftSum>0 and rightSum>0:
            self.maxValue = max(self.maxValue, root.val+leftSum+rightSum)
        maxValueUp = max(leftSum, rightSum) +root.val
        self.maxValue = max(self.maxValue, maxValueUp)
        return maxValueUp