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