返回树型数据怎么写？

 public List<OneSubject> getAllOneTwoSubject() {
        //查询以及分类
        QueryWrapper<EduSubject> wrapperOne = new QueryWrapper<>();
        wrapperOne.eq("parent_id", "0"); //mybatis-plus中的方法
        List<EduSubject> oneSubjectList = baseMapper.selectList(wrapperOne);
//        this.list(wrapperOne); //等同于baseMapper.selectList()

        //查询二级分类
        QueryWrapper<EduSubject> wrapperTwo = new QueryWrapper<>();
        wrapperOne.ne("parent_id", "0");
        List<EduSubject> twoSubjectList = baseMapper.selectList(wrapperTwo);

        //创建list集合，用于存放最终的数据
        ArrayList<OneSubject> finalSubjectList = new ArrayList<>();

        //封装一级分类
        //查询出来所有的一级分类list集合遍历，得到每个一级分类对象，获取每个一级分类对象值，
        //封装到要求的list集合里面 List<OneSubject> finalSubjectList

        for (int i = 0; i < oneSubjectList.size(); i++) {
            //得到oneSubjectList每个eduSubject对象
            EduSubject eduSubject = oneSubjectList.get(i);
            //把eduSubject里面值获取出来，放到OneSubject对象里面
            OneSubject oneSubject = new OneSubject();
//            oneSubject.setId(eduSubject.getId());
//            oneSubject.setTitle(eduSubject.getTitle());
            //eduSubject值复制到对应oneSubject对象里面
            BeanUtils.copyProperties(eduSubject, oneSubject);
            //多个OneSubject放到finalSubjectList里面
            finalSubjectList.add(oneSubject);

            //封装二级分类
            //在一级分类循环遍历查询所有的二级分类
            //创建list集合封装每个一级分类的二级分类
            List<TwoSubject> twoFinalSubjectList = new ArrayList<>();
            //遍历二级分类list集合
            for (int m = 0; m < twoSubjectList.size(); m++) {
                //获取每个二级分类
                EduSubject tSubject = twoSubjectList.get(m);
                //判断二级分类parentid和一级分类id是否一样
                if (tSubject.getParentId().equals(eduSubject.getId())) {
                    //把tSubject值复制到TwoSubject里面，放到twoFinalSubjectList里面
                    TwoSubject twoSubject = new TwoSubject();
                    BeanUtils.copyProperties(tSubject, twoSubject);
                    twoFinalSubjectList.add(twoSubject);
                }

            }
            //把一级下面所有二级分类放到一级分类里面
            oneSubject.setChildren(twoFinalSubjectList);

        }


        return finalSubjectList;
    }