LintCode Convert BST to Greater Tree

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.

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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;
    }
};