leetcode - Binary Tree Maximum Path Sum

Given a binary tree, find the maximum path sum.

The path may start and end at any node in the tree.

For example:
Given the below binary tree,

       1      / \     2   3

Return 6.

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */struct TreeNode{int val;TreeNode *left;TreeNode *right;TreeNode(int x) : val(x), left(NULL), right(NULL) {}};class Solution {public:    int maxPathSum(TreeNode *root) {if(root == NULL) return 0;maxNode = root->val;dfs(root);return maxNode;    }private:int maxNode;int dfs(TreeNode *root){if(root == NULL) return 0;int left = dfs(root->left);int right= dfs(root->right);int val = root->val;if(left > 0) val += left;if(right> 0) val += right;if(maxNode < val) maxNode = val;return std::max(std::max(left,right),0) + root->val;}};