private static List<Integer> sort(List<Integer> list) {
    if (list.size() == 0) return new ArrayList<>();

    List<Integer> smaller = new ArrayList<>();
    List<Integer> bigger = new ArrayList<>();
    int pivot = list.remove(0);

    for (Integer num : list) {
        if (num < pivot) {
            smaller.add(num);
        } else {
            bigger.add(num);
        }
    }

    list = sort(smaller);
    list.add(pivot);
    list.addAll(sort(bigger));

    return list;
}