leetcode @python 124. Binary Tree Maximum Path Sum

题目链接

https://leetcode.com/problems/binary-tree-maximum-path-sum/

题目原文

Given a binary tree, find the maximum path sum.

For this problem, a path is defined as any sequence of nodes from some starting node to any node in the tree along the parent-child connections. The path does not need to go through the root.

For example:
Given the below binary tree,

   1
  / 
 2   3

Return 6.

题目大意

以树的任意节点为起点或者重点，找到路径的最大值

解题思路

使用一个全局变量current_max，记录当前计算得出的最大路径。递归的思路：左子树中的最大路径和，右子树中的最大路径和，以及左子树中以root.left为起点的最大路径（需要大于零）+右子树中以root.right为起点的最大路径（需要大于零）+root.val），这三者中的最大值就是最大的路径和

代码

# 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):
    current_max = float('-inf')

    def maxPathSum(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        self.dp(root)
        return self.current_max

    def dp(self, root):
        if root == None:
            return 0
        left = self.dp(root.left)
        right = self.dp(root.right)

        if not left or left < 0:
            left = 0
        if not right or right < 0:
            right = 0
        self.current_max = max(left + right + root.val, self.current_max)
        return max(left, right) + root.val