Given the below binary tree:

  1
 / 2   3

return 6.

class Solution {
public:
    int maxPathSum(TreeNode *root) {
        if (root == NULL) {
            return 0;
        }
        
        int res = INT_MIN; 
        helper(root, res);
        
        return res;
    }
    
    int helper(TreeNode *root, int &res) {
        if (root == NULL) {
            return 0;
        }
        
        int sum = root->val;
        int leftMax = helper(root->left, res);
        int rightMax = helper(root->right, res);
        
        if (leftMax > 0) {
            sum += leftMax;
        }
        if (rightMax > 0) {
            sum += rightMax;
        }
        
        res = max(sum, res);
        
        return max(0, max(leftMax, rightMax)) + root->val;
    }    
};