LeetCode - Convert BST to Greater Tree

Given a Binary Search Tree (BST), convert it to a Greater Tree such that every key of the original BST is changed to the original key plus sum of all keys greater than the original key in BST.

Example:

Input: The root of a Binary Search Tree like this:
              5
            /              2     13

Output: The root of a Greater Tree like this:
             18
            /             20     13

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int sum = 0;
    public TreeNode convertBST(TreeNode root) {
        helper(root);
        return root;
    }
    
    public void helper (TreeNode root){
        if(root == null){
            return;
        }
        helper(root.right);
        sum = sum + root.val;
        root.val = sum;
        helper(root.left);
        
    }
}