// Textbook fragment 11.03
  /**
   * Sorts the elements of list in in nondecreasing order according
   * to comparator c, using the merge-sort algorithm.
   **/
  public static <E> void mergeSort (PositionList<E> in, Comparator<E> c) {
    int n = in.size();
    if (n < 2)
      return;  // the in list is already sorted in this case
    // divide
    PositionList<E> in1 = new NodePositionList<E>();
    PositionList<E> in2 = new NodePositionList<E>();
    int i = 0;
    while (i < n/2) {
      in1.addLast(in.remove(in.first())); // move the first n/2 elements to in1
      i++;
    }
    while (!in.isEmpty())
      in2.addLast(in.remove(in.first())); // move the rest to in2
    // recur
    mergeSort(in1,c);
    mergeSort(in2,c);
    //conquer
    merge(in1,in2,c,in);
  }