Qsort

package quicksort;
import java.util.ArrayList;

public class QuickSort {
    /**
     * This method sort the input ArrayList using quick sort algorithm.
     * @param input the ArrayList of integers.
     * @return sorted ArrayList of integers.
     */
    private ArrayList<Integer> quicksort(ArrayList<Integer> input){

        if(input.size() <= 1){
            return input;
        }

        int middle = (int) (input.size() / 2);
        int pivot = input.get(middle); //zwraca el o indexie

        ArrayList<Integer> less = new ArrayList<Integer>();
        ArrayList<Integer> greater = new ArrayList<Integer>();

        for (int i = 0; i < input.size(); i++) {
            if(input.get(i) <= pivot){
                if(i == middle){
                    continue;
                }
                less.add(input.get(i));
            }
            else{
                greater.add(input.get(i));
            }
        }
        return concatenate(quicksort(less), pivot, quicksort(greater));
    }

    /**
     * Join the less array, pivot integer, and greater array
     * to single array.
     * @param less integer ArrayList with values less than pivot.
     * @param pivot the pivot integer.
     * @param greater integer ArrayList with values greater than pivot.
     * @return the integer ArrayList after join.
     */
    private ArrayList<Integer> concatenate(ArrayList<Integer> less, int pivot, ArrayList<Integer> greater){

        ArrayList<Integer> list = new ArrayList<Integer>();

        for (int i = 0; i < less.size(); i++) {
            list.add(less.get(i));
        }
        list.add(pivot);
        for (int i = 0; i < greater.size(); i++) {
            list.add(greater.get(i));
        }
        return list;
    }

        public static void main(String[] args) {
        ArrayList<Integer> input = new ArrayList<>();
            input.add(5);
            input.add(6);
            input.add(3);
            input.add(2);
            input.add(8);
            input.add(-4);
            input.add(-9);
            input.add(9);
            input.add(0);
            input.add(1);

            QuickSort app = new QuickSort();
        System.out.println(input); //Before sort
        System.out.println(app.quicksort(input)); //After sort
    }
}