programming-examples/java/Data_Structures/Quicksort.java

import java.util.*;

public class Quicksort {

	static Random rnd = new Random();

	public static void quickSort(int[] a, int low, int high) {
		if (low >= high)
			return;
		int separator = a[low + rnd.nextInt(high - low + 1)];
		int i = low;
		int j = high;
		while (i <= j) {
			while (a[i] < separator)
				++i;
			while (a[j] > separator)
				--j;
			if (i <= j) {
				int t = a[i];
				a[i] = a[j];
				a[j] = t;
				++i;
				--j;
			}
		}
		quickSort(a, low, j);
		quickSort(a, i, high);
	}

	// test
	public static void main(String[] args) {
		int n = 10_000_000;
		int[] a1 = rnd.ints(n).toArray();

		int[] a2 = a1.clone();
		Arrays.sort(a2);

		long time = System.currentTimeMillis();
		quickSort(a1, 0, a1.length - 1);
		System.out.println(System.currentTimeMillis() - time);

		if (!Arrays.equals(a1, a2))
			throw new RuntimeException();
	}
}