0589-n-ary-tree-preorder-traversal.js

/**
 * 589. N-ary Tree Preorder Traversal
 * https://leetcode.com/problems/n-ary-tree-preorder-traversal/
 * Difficulty: Easy
 *
 * Given the root of an n-ary tree, return the preorder traversal of its nodes' values.
 *
 * Nary-Tree input serialization is represented in their level order traversal. Each group
 * of children is separated by the null value (See examples)
 */

/**
 * // Definition for a _Node.
 * function _Node(val, children) {
 *    this.val = val;
 *    this.children = children;
 * };
 */

/**
 * @param {_Node|null} root
 * @return {number[]}
 */
var preorder = function(root) {
  return !root ? [] : [root.val, ...root.children.flatMap(n => preorder(n))];
};