leetcode 538. Convert BST to Greater Tree(反向中序遍历)

问题描述：

  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.

思路：

  由于二叉搜索数的中序遍历的结果是从小到大排列的结点值，利用这个特性，采用反向中序遍历，从大向小检查数字，每个结点的值都加上已经遍历过的结点值的和，即可。

代码：

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