题目描述：
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.

您在真实的面试中是否遇到过这个题？ Yes
样例
Given a binary search Tree ｀{5,2,3}｀:

          5
        /   \
       2     13

Return the root of new tree

         18
        /   \
      20     13

思路分析：
后序遍历，后面的加上前面的val值。

ac代码：

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
class Solution {
public:
    /**
     * @param root the root of binary tree
     * @return the new root
     */
    int sum=0;
    void dfs(TreeNode *root)
    {
        if(root==NULL)
            return ;
        dfs(root->right);
        root->val=root->val+sum;
        sum=root->val;
        dfs(root->left);
    }
    TreeNode* convertBST(TreeNode* root) {
        // Write your code here
        dfs(root);
        return root;
    }
};