0669-trim-a-binary-search-tree.js

/**
 * 669. Trim a Binary Search Tree
 * https://leetcode.com/problems/trim-a-binary-search-tree/
 * Difficulty: Medium
 *
 * Given the root of a binary search tree and the lowest and highest boundaries as low and high,
 * trim the tree so that all its elements lies in [low, high]. Trimming the tree should not change
 * the relative structure of the elements that will remain in the tree (i.e., any node's descendant
 * should remain a descendant). It can be proven that there is a unique answer.
 *
 * Return the root of the trimmed binary search tree. Note that the root may change depending on
 * the given bounds.
 */

/**
 * 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
 * @param {number} low
 * @param {number} high
 * @return {TreeNode}
 */
var trimBST = function(root, low, high) {
  if (!root) return null;

  if (root.val < low) return trimBST(root.right, low, high);
  if (root.val > high) return trimBST(root.left, low, high);
  root.left = trimBST(root.left, low, high);
  root.right = trimBST(root.right, low, high);

  return root;
};