给定一棵二叉树的根节点root，返回这棵树的前序遍历数组。
    示例：
    144. 二叉树前序遍历 - 图1
    输入：root = [1, null, 2, 3]
    输出：[1, 2, 3]

    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 void flatten(TreeNode root) {
    18.         List<Integer> res = new ArrayList<>();
    19.         preorder(root, res);
    20.         return res;
    21.     }
    23.     private void preorder(TreeNode root, List<Integer> res) {
    24.         if (root == null) {
    25.             return;
    26.         }
    27.         res.add(root.val);
    28.         preorder(root.left, res);
    29.         preorder(root.right, res);
    30.     }
    31. }