MergeSort.java

import java.util.Arrays;

public class MergeSort {
    public static void main(String[] args) {
        int[] arr = {5,4,3,2,1};
        arr = mergeSort(arr);
        System.out.println(Arrays.toString(arr));
    }

    static int[] mergeSort(int[] arr){
        if(arr.length==1){
            return arr;
        }

        int mid = arr.length/2;

        int[] left = mergeSort(Arrays.copyOfRange(arr, 0, mid));
        int[] right = mergeSort(Arrays.copyOfRange(arr, mid, arr.length));

        return merge(left,right);
    }

    private static int[] merge(int[] first, int[] second) {
        int[] mix = new int[first.length+second.length];

        int i=0;
        int j=0;
        int k=0;

        while(i<first.length && j<second.length){
            if(first[i]<second[j]){
                mix[k]=first[i];
                i++;
            }else{
                mix[k]=second[j];
                j++;
            }
            k++;
        }

        // it is possible that one of the array is incomplete
        // copy the remaining elements
        while(i<first.length){
            mix[k]=first[i];
            i++;
            k++;
        }

        while(j<second.length){
            mix[k]=second[j];
            i++;
            k++;
        }

        return mix;
    }
}