这是一篇小白测试

package com.my.test;

import java.util.ArrayList;
import java.util.List;

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode() {}
    TreeNode(int val) { this.val = val; }
    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}

class Solution{
    public static void main(String[] args) {
        Integer[] a = new Integer[]{1,null,2,3};
        TreeNode root = new TreeNode();
        root = creatBiTree(root,a,0);
        System.out.println(inorderTraversal(root));
    }

    //中序遍历
    public static List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        if (root == null)
            return list;
        if (root.left != null)
            list.addAll(inorderTraversal(root.left));
        list.add(root.val);
        if (root.right != null)
            list.addAll(inorderTraversal(root.right));
        return list;
    }

    //创建二叉树
    public static TreeNode creatBiTree(TreeNode root,Integer[] a,int i){
        if (i < a.length){
            if (a[i] == null){
                root = null;
            }else {
                TreeNode lt = new TreeNode();
                TreeNode rt = new TreeNode();
                root.val = a[i];
                root.left = creatBiTree(lt,a,++i);
                root.right = creatBiTree(rt,a,++i);
            }
        }
        return root;
    }
}