[LeetCode]题解（python）：124-Binary Tree Maximum Path Sum

题目来源：

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

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题意分析：

　　给定一棵树，找出一个数值最大的路径，起点可以是任意节点或者叶子。

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题目思路：

　　我们可以先找路径的最大mr,ml，那么最大值是max（solve(root),solve(left),solve(right), max(mr + root.val + ml, root.val))。

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代码（python）：

　　

# 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
        """
        if root == None:
            return 0
        Solution.maxsum = root.val
        def solve(root):
            if root == None:
                return 0
            sum,l,r = root.val,0,0
            if root.left:
                l = solve(root.left)
                if l > 0: sum += l
            if root.right:
                r = solve(root.right)
                if r > 0: sum += r
            Solution.maxsum = max(Solution.maxsum,sum)
            #print(Solution.maxsum)
            return max(root.val,max(root.val + l,root.val+r))
        solve(root)
        return Solution.maxsum