144. Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes’ values.Example:Input: [1,null,2,3] 1 \ 2 / 3Output: [1,2,3]Follow up: Recursive solution is trivial, could you do it iteratively?难度：medium题目：给定二叉树，返回其前序遍历结点值。（不要使用递归）/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */class Solution { public List<Integer> preorderTraversal(TreeNode root) { Stack<TreeNode> stack = new Stack<>(); List<Integer> result = new ArrayList<>(); // root, left, right while (root != null || !stack.isEmpty()) { if (null == root) { root = stack.pop(); } result.add(root.val); if (root.right != null) { stack.push(root.right); } root = root.left; } return result; }}