// Textbook fragment 06.21
public class FavoriteListMTF<E> extends FavoriteList<E> {
  /** Default constructor */
  public FavoriteListMTF() { }
  /** Moves up an entry to the first position;  O(1) time  */
  protected void moveUp(Position<Entry<E>> pos) {
    fList.addFirst(fList.remove(pos));
  }
  /** Returns the k most accessed elements, for a given k; O(kn) time */
  public Iterable<E> top(int k) {
    if (k < 0 || k > size())
      throw new IllegalArgumentException("Invalid argument");
    PositionList<E> T = new NodePositionList<E>(); // top-k list
    if (!isEmpty()) {
      // copy entries into temporary list C
      PositionList<Entry<E>> C = new NodePositionList<Entry<E>>();
      for (Entry<E> e: fList)
        C.addLast(e);
      // find the top k elements, one at a time
      for (int i = 0; i < k; i++) {
        Position<Entry<E>> maxPos = null;       // position of top entry
        int maxCount = -1;                      // access count of top entry
        for (Position<Entry<E>> p: C.positions()) {
          // examine all entries of C
          int c = count(p);
          if (c > maxCount) {   // found an entry with higher access count
            maxCount = c;
            maxPos = p;
          }
        }
        T.addLast(value(maxPos));       // add top entry to list T
        C.remove(maxPos);               // remove top entry from list C
      }
    }
    return T;
  }
}