1038. Binary Search Tree to Greater Sum 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:
    void GST(TreeNode* root, int &sum)
    {
        if(root == NULL)
        {
            return;
        }

        GST(root -> right, sum);

        sum += root -> val;

        root -> val = sum;

        GST(root -> left, sum);
    }


    TreeNode* bstToGst(TreeNode* root) {
        int sum = 0;

        GST(root, sum);


        return root;
    }
};