void mergeSort (apvector<int> &list, int first, int last)

//  Recursively divides a list in half, over and over.  When the
//  sublist has one or two values, stop subdividing.

{
	if (first-last==1)
		// do nothing
	else
		if(first-last==2)
			{
					 if (list[first]>list[last])
							swap	(list[first],list[last]);
			}

		else
				{

			  last=last/2;;	// recursion, divide list into two halves
									//	Find midpoint of current sublist
						mergeSort(list,first,last);   //Call mergeSort and process left sublist
						mergeSort(list,first,last);   //Call mergeSort and process right sublist
						merge(list,first,last);      // merge left and right sublists

				}
}