public List<Integer> sort(List<Integer> input) {
        if (input.size() <= 1) return input;
        else {
            Integer pivot = input.get(input.size() / 2);
            Stream<Integer> smallerThanPivot = input.stream().filter(i -> i.compareTo(pivot) == -1);
            Stream<Integer> equalToPivot = input.stream().filter(i -> i.equals(pivot));
            Stream<Integer> biggerThanPivot = input.stream().filter(i -> i.compareTo(pivot) == 1);
            return sort(concat(concat(smallerThanPivot, equalToPivot), biggerThanPivot).collect(toList()));
        }
    }