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

Given a binary search Tree ｀{5,2,3}｀:

              5
            /   \
           2     13

Return the root of new tree

             18
            /   \
          20     13

逆中序遍历树，注意root的greatSum是input greatSum和右子树sum的和；输入给左子树的greatSum 是root.val

 /**
  * Definition of TreeNode:
  * public class TreeNode {
  *     public int val;
  *     public TreeNode left, right;
  *     public TreeNode(int val) {
  *         this.val = val;
  *         this.left = this.right = null;
  *     }
  * }
  */
 public class Solution {
     /**
      * @param root the root of binary tree
      * @return the new root
      */
     public TreeNode convertBST(TreeNode root) {
         // Write your code here
         if (root == null) {
             return root;
         }
         helper(root, 0);
         return root;
     }
     private int helper(TreeNode root, int greatSum){
         int sum = 0;
         if (root.right != null) {
             sum += helper(root.right, greatSum);
         }
         int temp = greatSum + sum;
         sum += root.val;
         root.val += temp;
         if (root.left != null) {
             sum += helper(root.left, root.val);
         }
         return sum;
     }
 }