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     13Output: 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；

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 class Solution{11     private int sum = 0;12     public TreeNode convertBST(TreeNode root) {13        helper(root);14        return root;      15     }        16     public void helper(TreeNode root){17         if (root!=null){18             helper(root.right);19             root.val+=sum;20             sum = root.val;21             helper(root.left);22         }23     }24 }