树形结构后端

//课程分类列表
    @ApiOperation(value = "课程分类列表")
    @PostMapping("getAllSubject")
    public R getAllSubject() {
        //list集合泛型是一级分类，里面已经包含了二级分类
        List list = subjectService.getAlloneTwoSubject();
        return R.ok().data("list",list);
    }

//查询所有的一级分类 parentId = 0
        QueryWrapper wrapperOne = new QueryWrapper<>();
        wrapperOne.eq("parent_id","0");
        List oneSubjectList = baseMapper.selectList(wrapperOne);

        //查询所有的二级分类 parentId != 0
        QueryWrapper wrapperTwo = new QueryWrapper<>();
        wrapperTwo.ne("parent_id","0");
        List twoSubjectList = baseMapper.selectList(wrapperTwo);

 List finalSubjectList = new ArrayList<>();

//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 finalSubjectList
        for (int i = 0; i < oneSubjectList.size(); i++) { //遍历oneSubjectList集合
            //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);

            //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 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;