Merge Sort

import java.util.Scanner;
import java.util.Random;
public class MergeSort {
	
		public static void main(String[] args) 
		{
			@SuppressWarnings("resource")
			Scanner scan=new Scanner(System.in);
			Random ran=new Random();
			long start,stop;
			System.out.println("Enter no of elements");
			int n=scan.nextInt();
			int[] a=new int[n];
	System.out.println(" Enter the choice 1: Best Case  2: Average Case  3:  Worst Case");
			int ch=scan.nextInt();
			switch(ch)
			{
				case 1: System.out.println(" Best Case");
						for(int i=0;i<n;i++)
							a[i]=i;
					break;
			   	case 2: System.out.println(" Average Case");
						for(int i=0;i<n;i++)
							a[i]=ran.nextInt(n);
					break;
			   	case 3: System.out.println(" Worst Case");
						for(int i=0;i<n;i++)
							a[i]=n-i;
					break;		
				
			}// end switch
	//recording the start time
			start=System.nanoTime();
			//function call
			Mergesort(a,0,n-1);
			// recording the end time
			stop=System.nanoTime();
			display(a);
	System.out.println("\nTime taken to sort " +a.length+ " elements =" +(stop-start));

		}// end main
			
		private static void display(int[] a)
		{
			// TODO Auto-generated method stub
			System.out.println("the sorted array is");
			for(int i=0;i<a.length;i++)
				System.out.println(a[i]);
			
		}//end display

		//function o divide the array
		public static void Mergesort(int[] a, int low, int high)
		{
			int mid;
			if(low<high)// array contains more than one element
			{
				mid=(low+high)/2;// dividing the array in to two sub arrays
				Mergesort(a, low, mid);// sorting sub arrays
				Mergesort(a, mid+1, high);
				Merge(a,low,mid,high);// combining or merging the sorted arrays
			}
		}// end Mergesort

		//function to merge two sorted arrays
		public static void Merge(int[] arr,int low,int mid,int high) 
		{
			int k,h=low,i=low,j=mid+1;
			int[] b=new int[arr.length];
			while(h<=mid && j<=high)
			{
				if(arr[h]<=arr[j])
				{
					b[i]=arr[h];
					h++;
				}
				else
				{
					b[i]=arr[j];
					j++;
				}
				i++;
			}// end while
			if(h>mid) // for remaining elements in upper half
			{
				for(k=j;k<=high;k++)
				{
					b[i]=arr[k];  
					i++;
				}
			}
			else // for remaining elements in lower half
			{
				for(k=h;k<=mid;k++)
				{
					b[i]=arr[k];
					i++;
				}		
			}
			//copy the contents from auxiliary array i.e. from b to arr 
			for(k=low;k<=high;k++)
				arr[k]=b[k];
		}// end merge
	}// end MergeSort class