Best Case, O(nlogn)

Worst Case, Ω(nlogn)

Memory Complexity, O(n)

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

public class Merge
{
    // Merges two subarrays of arr[].
    // First subarray is arr[l..m]
    // Second subarray is arr[m+1..r]
    public static void merge(int arr[], int l, int m, int r)
    {
        // Find sizes of two subarrays to be merged
        int n1 = m - l + 1;
        int n2 = r - m;
 
        /* Create temp arrays */
        int L[] = new int [n1];
        int R[] = new int [n2];
 
        /*Copy data to temp arrays*/
        for (int i=0; i<n1; ++i)
            L[i] = arr[l + i];
        for (int j=0; j<n2; ++j)
            R[j] = arr[m + 1+ j];
 
 
        /* Merge the temp arrays */
 
        // Initial indexes of first and second subarrays
        int i = 0, j = 0;
 
        // Initial index of merged subarry array
        int k = l;
        while (i < n1 && j < n2)
        {
            if (L[i] <= R[j])
            {
                arr[k] = L[i];
                i++;
            }
            else
            {
                arr[k] = R[j];
                j++;
            }
            k++;
        }
 
        /* Copy remaining elements of L[] if any */
        while (i < n1)
        {
            arr[k] = L[i];
            i++;
            k++;
        }
 
        /* Copy remaining elements of R[] if any */
        while (j < n2)
        {
            arr[k] = R[j];
            j++;
            k++;
        }
    }

    // merge()
    public static void sort(int arr[], int l, int r)
    {
        if (l < r)
        {
            // Find the middle point
            int m = (l+r)/2;

            // Sort first and second halves
            sort(arr, l, m);
            sort(arr , m+1, r);

            // Merge the sorted halves
            merge(arr, l, m, r);
        }
    }

    //generate an array with random elements
    public static int [] getArray(int size)
    {
	    int [] array = new int[size];
	    Random r = new Random();

	    for(int i = 0; i < size; i++)
		    array[i] = r.nextInt(100);

	    return array;
    }

    public static void main(String args[])
    {
	    long start, end;
	    int [] array;
	    int [] array2;
	    int size = 100000;

	    //create two same arrays
	    array = getArray(size);
	    array2 = Arrays.copyOf(array, array.length);

	    //sort a huge array with bubble sort
	    start = System.currentTimeMillis();
	    sort(array, 0, array.length-1);
	    end = System.currentTimeMillis();
	    System.out.printf("Bubble sort time: %10f%n", (end-start)/1000000.0);

	    //sort the array with build-in sort function in Arrays class
	    start = System.currentTimeMillis();
	    Arrays.sort(array2);
	    end = System.currentTimeMillis();
	    System.out.printf("Build-in sort time: %10f%n", (end-start)/1000000.0);

	    //validate merge sort
	    if(Arrays.equals(array, array2))
		    System.out.println("Merge sort is validated ...");
	    else
		    System.out.println("Merge sort does not work correctly ...");
    }
}