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);

    }

}