Quick Sort

import java.util.Random;
import java.util.Scanner;
public class QuickSort {


		static int[] a = new int[500000];
		static int n;
		private static Scanner s;

		public static void main(String[] args) 
		{
			Random r = new Random();
			s = new Scanner(System.in);
			System.out.println("Enter the number of elements:");
			n = s.nextInt();
			int i;
			System.out.println("1. Best Case\n2. Average Case\n3. Worst Case");
			int ch = s.nextInt();
			switch(ch) 
			{
					case 1: for(i=0;i<n;i++)
						a[i] = r.nextInt(1000);
								
					break;
					case 2: for(i=0;i<n;i++)
						a[i] = n - i;	
								break;
					case 3: for(i=0;i<n;i++)
								a[i] = i;
					break;
			}
			long start = System.nanoTime();
			quickSort(0,n-1);
			long end = System.nanoTime();
			System.out.println("the sorted array is");
				for(i=0;i<n;i++)
					System.out.println(a[i]);
			System.out.println("Time elapsed: " + (end - start));
		}

		public static void quickSort(int low, int high)
		{
			if(low<high)
			{
				int j = partition(low,high+1);
				quickSort(low,j-1);
				quickSort(j+1,high);
			}
		}

		public static int partition(int low, int high)
		{
			int pivot = a[low];
			int i = low;
			int j = high;
			do	{
				do	{
						i++;
					}while(a[i]<pivot && i<high);
				do	{
						j--;
					}while(a[j]>pivot && j>low);
				if(i<j)
					{
						int temp = a[i];
						a[i] = a[j];
						a[j] = temp;
					}	
				} while(i<j);
			a[low]=a[j];
			a[j]=pivot;
			return j;
		}
	}