/**

 * 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 maxPathSum(TreeNode* root) {

        int res = INT_MIN;

        DFS(root,res);

        return res;

    }

    int DFS(TreeNode* root,int &res){

        if(!root) return ;

        int left = max(DFS(root->left,res),);

        int right = max(DFS(root->right,res),);

        res = max(res,left+right+root->val);

        return max(left,right)+root->val; //res是整个树的最大值,return 的是作为单节点的最大值，不矛盾

    }

};