给定一个二叉树的根节点 root ，返回它的 中序 遍历。
    示例：

    94. 二叉树中序遍历 - 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 List<Integer> inorderTraversal(TreeNode root) {
    18.         List<Integer> list = new ArrayList<>();
    19.         inorder(root, list);
    20.         return list;
    21.     }
    23.     private void inorder(TreeNode node, List<Integer> res) {
    24.         if (node == null) {
    25.             return;
    26.         }
    27.         inorder(node.left, res);
    28.         res.add(node.val);
    29.         inorder(node.right, res);
    30.     }
    31. }