class DLLNode<E> {
	protected E element;
	protected DLLNode<E> pred, succ;

	public DLLNode(E elem, DLLNode<E> pred, DLLNode<E> succ) {
		this.element = elem;
		this.pred = pred;
		this.succ = succ;
	}

	@Override
	public String toString() {
		return "<-"+element.toString()+"->";
	}
}


class DLL<E> {
	private DLLNode<E> first, last;

	public DLL() {
		// Construct an empty SLL
		this.first = null;
		this.last = null;
	}

	public void deleteList() {
		first = null;
		last = null;
	}

	public int length() {
		int ret;
		if (first != null) {
			DLLNode<E> tmp = first;
			ret = 1;
			while (tmp.succ != null) {
				tmp = tmp.succ;
				ret++;
			}
			return ret;
		} else
			return 0;

	}

	public void insertFirst(E o) {
		DLLNode<E> ins = new DLLNode<E>(o, null, first);
		if (first == null)
			last = ins;
		else
			first.pred = ins;
		first = ins;
	}

	public void insertLast(E o) {
		if (first == null)
			insertFirst(o);
		else {
			DLLNode<E> ins = new DLLNode<E>(o, last, null);
			last.succ = ins;
			last = ins;
		}
	}

	public void insertAfter(E o, DLLNode<E> after) {
		if(after==last){
			insertLast(o);
			return;
		}
		DLLNode<E> ins = new DLLNode<E>(o, after, after.succ);
		after.succ.pred = ins;
		after.succ = ins;
	}

	public void insertBefore(E o, DLLNode<E> before) {
		if(before == first){
			insertFirst(o);
			return;
		}
		DLLNode<E> ins = new DLLNode<E>(o, before.pred, before);
		before.pred.succ = ins;
		before.pred = ins;
	}

	public E deleteFirst() {
		if (first != null) {
			DLLNode<E> tmp = first;
			first = first.succ;
			first.pred = null;
			if (first == null)
				last = null;
			return tmp.element;
		} else
			return null;
	}

	public E deleteLast() {
		if (first != null) {
			if (first.succ == null)
				return deleteFirst();
			else {
				DLLNode<E> tmp = last;
				last = last.pred;
				last.succ = null;
				return tmp.element;
			}
		}
		// else throw Exception
		return null;
	}


	@Override
	public String toString() {
		String ret = new String();
		if (first != null) {
			DLLNode<E> tmp = first;
			ret += tmp + "<->";
			while (tmp.succ != null) {
				tmp = tmp.succ;
				ret += tmp + "<->";
			}
		} else
			ret = "Prazna lista!!!";
		return ret;
	}

	public DLLNode<E> getFirst() {
		return first;
	}

	public DLLNode<E> getLast() {

		return last;
	}

}