給你二叉樹的根節點 root ，返回它節點值的 前序 遍歷。

示例 1

輸入：root = [1,null,2,3]
輸出：[1,2,3]
示例 2：

輸入：root = []
輸出：[]
示例 3：

輸入：root = [1]
輸出：[1]
示例 4：

輸入：root = [1,2]
輸出：[1,2]
示例 5：

輸入：root = [1,null,2]
輸出：[1,2]

來源：力扣（LeetCode）
連結：https://leetcode-cn.com/problems/binary-tree-preorder-traversal

程式碼：

 1 /**
 2  * Definition for a binary tree node.
 
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode() {}
 8  *     TreeNode(int val) { this.val = val; }
 9  *     TreeNode(int val, TreeNode left, TreeNode right) {
10  *         this.val = val;
11  *         this.left = left;
12  *         this.right = right;
 
13  *     }
14  * }
15  */
16 class Solution {
17     public List<Integer> preorderTraversal(TreeNode root) {
18         List<Integer> res = new ArrayList<>();
19         if(root == null) return res;
20 
21         Stack<TreeNode> stack = new Stack<>();
22         stack.push(root);
 
23 
24         while(!stack.isEmpty()){
25             TreeNode node = stack.pop();
26             res.add(node.val);
27             if(node.right != null) stack.push(node.right);
28             if(node.left != null) stack.push(node.left);
29         }
30         return res;
31     }
32 }