Quick Sort

Solution.java

import java.util.Arrays;
import java.util.Random;
import java.util.Scanner;

class Solution {
    private void swap(int[] numbers, int i, int j) {
        int temp = numbers[i];
        numbers[i] = numbers[j];
        numbers[j] = temp;
    }

    private int getPartitionIndex(int[] numbers, int start, int end) {
        int randomIndex = (new Random()).nextInt(end - start) + start;
        swap(numbers, randomIndex, end);
        int partition = numbers[end];

        int i = start - 1;
        for(int j = start; j <= end - 1; j++) {
            if(numbers[j] <= partition) {
                swap(numbers, ++i, j);
            }
        }
        swap(numbers, ++i, end);
        return i;
    }

    private void sort(int[] numbers, int start, int end) {
        if(start >= end) {
            return;
        }

        int partitionIndex = getPartitionIndex(numbers, start, end);
        System.out.printf("start = %d, end = %d, partitionIndex = %d\n", start, end, partitionIndex);
        sort(numbers, start, partitionIndex - 1);
        sort(numbers, partitionIndex + 1, end);
    }

    private void sort(int[] numbers) {
        sort(numbers, 0, numbers.length - 1);
    }

    public Solution() {
        Scanner reader = new Scanner(System.in);

        int[] numbers = new int[20];
        for(int i = 0; i < numbers.length; i++) {
            numbers[i] = (new Random()).nextInt(100);
        }
        System.out.println(Arrays.toString(numbers));

        sort(numbers);

        System.out.println(Arrays.toString(numbers));
    }

    public static void main(String[] args) {
        new Solution();
    }
}