MergerSort.java

import java.util.*;
 
public class MergerSort 
{
    public static void main(String[] args) 
    {
        //Unsorted array
        Integer[] a = { 2, 6, 3, 5, 1, };
         
        //Call merge sort
        mergeSort(a);
         
        //Check the output which is sorted array
        System.out.println(Arrays.toString(a));
    }
 
    public static Comparable[] mergeSort(Comparable[] list) 
    {
        //If list is empty; no need to do anything
        if (list.length <= 1) {
            return list;
        }
         
        //Split the array in half in two parts
        Comparable[] first = new Comparable[list.length / 2];
        Comparable[] second = new Comparable[list.length - first.length];
        System.arraycopy(list, 0, first, 0, first.length);
        System.arraycopy(list, first.length, second, 0, second.length);
         
        //Sort each half recursively
        mergeSort(first);
        mergeSort(second);
         
        //Merge both halves together, overwriting to original array
        merge(first, second, list);
        return list;
    }
     
    private static void merge(Comparable[] first, Comparable[] second, Comparable[] result) 
    {
        //Index Position in first array - starting with first element
        int iFirst = 0;
         
        //Index Position in second array - starting with first element
        int iSecond = 0;
         
        //Index Position in merged array - starting with first position
        int iMerged = 0;
         
        //Compare elements at iFirst and iSecond, 
        //and move smaller element at iMerged
        while (iFirst < first.length && iSecond < second.length) 
        {
            if (first[iFirst].compareTo(second[iSecond]) < 0) 
            {
                result[iMerged] = first[iFirst];
                iFirst++;
            } 
            else
            {
                result[iMerged] = second[iSecond];
                iSecond++;
            }
            iMerged++;
        }
        //copy remaining elements from both halves - each half will have already sorted elements
        System.arraycopy(first, iFirst, result, iMerged, first.length - iFirst);
        System.arraycopy(second, iSecond, result, iMerged, second.length - iSecond);
    }
}