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; }
 * }
 */
public class Solution {
    public TreeNode convertBST(TreeNode root) {
        convertBST(root, 0);
        return root;
    }
    
    private int convertBST(TreeNode node, int sumOfNode) {
        if (node == null) return sumOfNode;
        
        int right = convertBST(node.right, sumOfNode);
        int left = convertBST(node.left, node.val + right);
        node.val += right;
        return left;
    }
}