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

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public TreeNode convertBST(TreeNode root) {
12         convertBST(root, 0);
13         return root;
14     }
15     
16     private int convertBST(TreeNode node, int sumOfNode) {
17         if (node == null) return sumOfNode;
18         
19         int right = convertBST(node.right, sumOfNode);
20         int left = convertBST(node.left, node.val + right);
21         node.val += right;
22         return left;
23     }
24 }