public void selectCourseHour(List<List<CourseHour>> candidateList, List<CourseHour> selected, List<List<CourseHour>> results, int target) {

        if (CollectionUtils.isEmpty(candidateList)){
            return;
        }

        List<CourseHour> candidate = candidateList.get(0);

        for (CourseHour oneCandidate : candidate){
            if (selected.contains(oneCandidate)) continue;

            List<CourseHour> currentSelected = new ArrayList<>();
            currentSelected.addAll(selected);
            currentSelected.add(oneCandidate);

            if (currentSelected.size() >= target){
                results.add(currentSelected);
            }

            this.selectCourseHour(candidateList.subList(1, candidateList.size()), currentSelected, results, target);
        }

        return;
    }