538.convert-bst-to-greater-tree.js

/*
 * @lc app=leetcode id=538 lang=javascript
 *
 * [538] Convert BST to Greater Tree
 *
 * https://leetcode.com/problems/convert-bst-to-greater-tree/description/
 *
 * algorithms
 * Easy (50.04%)
 * Total Accepted:    75.4K
 * Total Submissions: 149K
 * Testcase Example:  '[5,2,13]'
 *
 * 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.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {TreeNode}
 */
var convertBST = function(root) {
  let res = 0;
  function r(root) {
    if (root === null) return null;

    r(root.right);

    root.val += res;

    res = +root.val;

    r(root.left);

    return root;
  }
  r(root);
  return root;
};