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）：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) 
 * };
 */
class Solution {
private:
    int sum = 0 ;
public:
    void travel(TreeNode* root){
        if (!root)
            return ;
        if (root->right)
            travel(root->right) ;
        sum += root->val ;
        root->val = sum ;
        if (root->left)
            travel(root->left) ;
    }
    
    TreeNode* convertBST(TreeNode* root) {
        travel(root) ;
        return root ;
    }
};