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





Since this is a BST, we can do a reverse inorder traversal to traverse the nodes of the tree in descending order. In the process, we keep track of the running sum of all nodes which we have traversed thus far.

public class Solution {

    int sum = 0;
    
    public TreeNode convertBST(TreeNode root) {
        convert(root);
        return root;
    }
    
    public void convert(TreeNode cur) {
        if (cur == null) return;
        convert(cur.right);
        cur.val += sum;
        sum = cur.val;
        convert(cur.left);
    }
    
}