124. Binary Tree Maximum Path Sum

124. Binary Tree Maximum Path Sum

搞一个全局变量，存结果，一个辅助函数，返回当前节点的一条路径的最大和，是左右子节点最大值加上当前节点，过程中要更新全局变量

class Solution(object):
    def maxPathSum(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        self.res = float('-inf')
        self.helper(root)
        return self.res
    def helper(self, root):
        if root == None:
            return 0
        left = max(0, self.helper(root.left))
        right = max(0, self.helper(root.right))
        self.res = max(left + right + root.val, self.res)
        return max(left, right) + root.val