Binary Tree Preorder Traversal

Description

Given a binary tree, return the preorder traversal of its nodes' values.

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3
Output: [1,2,3]

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

Solution(javascript)

/*
 * @lc app=leetcode id=144 lang=javascript
 *
 * [144] Binary Tree Preorder Traversal
 */

// @lc code=start
/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
const preorderTraversal = function (root) {
  const stack = [root]
  const result = []
  while (stack.length > 0) {
    const node = stack.pop()
    if (node) {
      result.push(node.val)
      stack.push(node.right, node.left)
    }
  }
  return result
}