Convert BST to Greater Tree

Description

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

Note: This question is the same as 1038: https://leetcode.com/problems/binary-search-tree-to-greater-sum-tree/

Solution(javascript)

const convertBST = (root) => {
  let sum = 0
  const aux = (node) => {
    if (node) {
      aux(node.right)
      sum += node.val
      node.val = sum
      aux(node.left)
    }
  }
  aux(root)
  return root
}