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


将一棵二叉搜索树变成一棵更大的二叉搜索树


C++（40ms）：

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) 
 8  * };
 9  */
10 class Solution {
11 private:
12     int sum = 0 ;
13 public:
14     void travel(TreeNode* root){
15         if (!root)
16             return ;
17         if (root->right)
18             travel(root->right) ;
19         sum += root->val ;
20         root->val = sum ;
21         if (root->left)
22             travel(root->left) ;
23     }
24     
25     TreeNode* convertBST(TreeNode* root) {
26         travel(root) ;
27         return root ;
28     }
29 };