public void mergeSort(ArrayList <Comparable> a, int first, int last)
{
    if((last-first)==1)
    {
    }
    else if((last-first)==2)
    {
        if((a.get(first).compareTo(a.get(last))>0))
        {
            a.add(first, a.remove(last));
        }
    }
    else
    {
        mergeSort(a, first, ((last+first)/2));
        mergeSort(a, ((last+first)/2+1), last);
        merge(a,first,((last+first)/2), last);
    }
}
// creates a copy of ArrayList list called temp, and uses temp’s values to properly merge(sort) list
// from first to last
public void merge(ArrayList<Comparable> list, int first, int mid, int last)
{
    List<Comparable> copy = new ArrayList<Comparable>();
    for(int i = 0; i<list.size(); i++)
    {
        copy.add(list.get(i));
    }
    int index1=first;
    int index2=mid+1;
    for(int i = first; i<=last; i++)
    {
        if(index1>mid)
        {
            list.set(i, copy.get(index2));
            index2++;
        }
        else if(index2 >last)
        {
            list.set(i, copy.get(index1));
            index1++;
        }
        else if(copy.get(index1).compareTo(copy.get(index2))<0)
        {
            list.set(i, copy.get(index1));
            index1++;
        }
        else
        {
            list.set(i, copy.get(index2));
            index2++;
        }
    }

}