maximum sum path in binary tree

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int mp(TreeNode* root, int &maxi)
    {
        if(root == NULL)return 0;
        
        int l = max(0,mp(root->left,maxi));
        int r = max(0,mp(root->right,maxi));
        
        maxi = max(maxi, l+r+ root->val);
        
        return root->val + max(l,r);
    }
    int maxPathSum(TreeNode* root) {
        
        if(root == NULL) return 0;
        
        int a = INT_MIN;
        int b = mp(root,a);
        return a;
    }
};