边工作边刷题：70天一遍leetcode: day 14-1

Binary Tree Maximum Path Sum

错误点：negative也可能是最大值，所以global maxVal初始值为INT_MIN。但是左右传上来的一侧值（left or right）一定要先和0比较再加到local maxVal做比较，因为如果left or right是negative，那么root可以选择不用。

# 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 maxPathSum(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        self.maxSum = -sys.maxint-1
        if not root: return 0
        self.maxdfs(root)
        return self.maxSum
        
    def maxdfs(self, root):
        leftMax = 0 
        if root.left:
            leftMax = max(0, self.maxdfs(root.left))
            
        rightMax = 0
        if root.right:
            rightMax = max(0, self.maxdfs(root.right))
        
        if leftMax + rightMax + root.val>self.maxSum:
            self.maxSum = leftMax + rightMax + root.val
        
        return max(leftMax+root.val, rightMax+root.val, root.val)