Binary Tree Preorder Traversal

这么简单的基本功能不能一次过啊！！！

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public List<Integer> preorderTraversal(TreeNode root) {        List<Integer> list = new LinkedList<>();        if (root == null) {            return list;        }        Stack<TreeNode> stack = new Stack<>();        stack.push(root);        while (!stack.isEmpty()) {            TreeNode node = stack.pop();            //list.add(node);            list.add(node.val);            if (node.right != null) {                stack.push(node.right);            }            if (node.left != null) {                stack.push(node.left);            }        }        return list;    }}