[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) {}
 * };
 */
class Solution {
private:
    int ans;
public:
    int DFS(TreeNode *root)
    {
        if(root==NULL) return 0;
        else
        {
            int sum=root->val;
            int sum1=DFS(root->left);
            int sum2=DFS(root->right);
            if(sum1>0) sum+=sum1;
            if(sum2>0) sum+=sum2;
            ans=max(ans,sum);
            return max(root->val,max(root->val+sum1,root->val+sum2));
        }
    }
    int maxPathSum(TreeNode *root) {
        if(root==NULL) return 0;
        ans=root->val;
        return max(ans,DFS(root));
    }
};