[Leetcode]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

思路：中序遍历；然后关键点在于用了一个全局变量；

 /**
  * Definition for a binary tree node.
  * public class TreeNode {
  *     int val;
  *     TreeNode left;
  *     TreeNode right;
  *     TreeNode(int x) { val = x; }
  * }
  */
 class Solution{
     private int sum = 0;
     public TreeNode convertBST(TreeNode root) {
        helper(root);
        return root;
     }
     public void helper(TreeNode root){
         if (root!=null){
             helper(root.right);
             root.val+=sum;
             sum = root.val;
             helper(root.left);
         }
     }
 }