589. N-ary Tree Preorder Traversal

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)

Example 1:

Input: root = [1,null,3,2,4,null,5,6] Output: [1,3,5,6,2,4]
Example 2:

Input: root = [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14] Output: [1,2,3,6,7,11,14,4,8,12,5,9,13,10]

Constraints:

• The number of nodes in the tree is in the range [0, 104].
• 0 <= Node.val <= 104
• The height of the n-ary tree is less than or equal to 1000.

Follow up: Recursive solution is trivial, could you do it iteratively?

Runtime: 96 ms, faster than 75.51% of JavaScript online submissions for N-ary Tree Preorder Traversal.
Memory Usage: 42 MB, less than 76.89% of JavaScript online submissions for N-ary Tree Preorder Traversal.

/**
 * // Definition for a Node.
 * function Node(val, children) {
 *    this.val = val;
 *    this.children = children;
 * };
 */
/**
 * @param {Node|null} root
 * @return {number[]}
 */
var preorder = function(root) {
    const stack = [];
    const result = [];
    let node = root;
    let children = [];
    while(stack.length > 0 || node) {
        if (node) {
            result.push(node.val);
            children = node.children || [];
            node = children[0];
            for (let i = children.length - 1; i > 0; i--) {
                stack.push(children[i]);
            }
        } else {
            node = stack.pop();
        }
    }
    return result;
};