1038-binary-search-tree-to-greater-sum-tree.js

/**
 * 1038. Binary Search Tree to Greater Sum Tree
 * https://leetcode.com/problems/binary-search-tree-to-greater-sum-tree/
 * Difficulty: Medium
 *
 * Given the root of 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 the sum of all keys greater
 * than the original key in BST.
 *
 * As a reminder, a binary search tree is a tree that satisfies these constraints:
 * - The left subtree of a node contains only nodes with keys less than the node's key.
 * - The right subtree of a node contains only nodes with keys greater than the node's key.
 * - Both the left and right subtrees must also be binary search trees.
 */

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {TreeNode}
 */
var bstToGst = function(root) {
  let sum = 0;
  traverse(root);
  return root;

  function traverse(node) {
    if (!node) return;
    traverse(node.right);
    sum += node.val;
    node.val = sum;
    traverse(node.left);
  }
};