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

/**
* 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 {
public:
void convert(TreeNode* root, int& sum){
if(root == NULL)    return;
convert(root->right, sum);
root->val += sum;
sum = root->val;
convert(root->left, sum);
}

TreeNode* convertBST(TreeNode* root) {
int sum = 0;
convert(root, sum);
return root;
}
};