538. 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 class Solution {
 2     int sum = 0;
 3     public TreeNode convertBST(TreeNode root) {
 4         convert(root);
 5         return root;
 6     }
 7     public void convert(TreeNode root){
 8         if(root==null) return;
 9         convert(root.right);
10         root.val+=sum;
11         sum = root.val;
12         convert(root.left);
13     } 
14 }