index.js

/*
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
*/

function convertToGreaterTree(tree, sum = 0/*move the sum out of func, It will
 reduce space complexity*/) {
  if (!tree) return sum;

  sum = tree.val += convertToGreaterTree(tree.right, sum);
  sum = convertToGreaterTree(tree.left, sum);

  return sum;
}

function TreeNode(val, left = null, right = null) {
  this.val = val;
  this.left = left;
  this.right = right;
}