private void combineHelper(int n, int k, int start, ArrayList<Integer> solution, ArrayList<ArrayList<Integer>> results) {
        if (solution.size() == k) {
            results.add(new ArrayList<Integer>(solution));
            return;
        }
        else if (start > n) {
            return;
        }
        for (int i = start; i <= n; i++) {
            solution.add(i);
            combineHelper(n, k, i + 1, solution, results);
            solution.remove(solution.size() - 1);
        }
        return;
    }