Leetcode之算法专题《Binary Tree Maximum Path Sum》

题目内容如下（链接：https://leetcode.com/problems/binary-tree-maximum-path-sum/description/）

首先说明本人代码运行速度打败了99%的人，见下图

Given a non-empty 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 must contain at least one node and does not need to go through the root.

Example 1:

Input: [1,2,3]

       1
      / \
     2   3

Output: 6

Example 2:

Input: [-10,9,20,null,null,15,7]

   -10
   / \
  9  20
    /  \
   15   7

Output: 42

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int dfs(TreeNode* root, int& ans){
        if(!root)
        {
            return 0;
        }
        int leftSum = dfs(root->left, ans);
        int rightSum = dfs(root->right, ans);
        if(leftSum >= 0 && rightSum >= 0)
        {
            ans = max(ans, root->val + leftSum + rightSum);
        }
        int ret_val = root->val + max(max(leftSum, rightSum), 0);
        ans = max(ans, ret_val);
        return ret_val;
    }
    int maxPathSum(TreeNode* root) {
        if(!root)
        {
            return 0;
        }
        int ans = root->val;
        dfs(root, ans);
        return ans;
    }
};