Largest subtree sum in a tree

class Solution {
  public:
    // Function to find largest subtree sum.
    int helper(Node * root , int &res)
    {
        if(root == NULL)
        {
            return 0 ; 
        }
        int l = helper(root->left , res) ;
        int r = helper(root->right , res) ;
        res = max(res , l + r + root->data) ;
        return l + r + root->data ; 
    }
    int findLargestSubtreeSum(Node* root)
    {
        //Write your code here
        int res = -1e9 ; 
        helper(root , res) ;
        return res ; 
    }
};