import java.util.Iterator;


public class NLinkedList<E> implements Iterable<E> {
	
	private NLinkedListNode<E> head, tail;
	public NLinkedListNode<E> getFirst(){ return this.head; }
	public NLinkedListNode<E> getLast(){ return this.tail; }
	
	public NLinkedList(){
		head = null;
		tail = null;
	}
	
	public void addFirst(E value){
		this.addFirst(new NLinkedListNode<E>(value));
	}
	public void addFirst(NLinkedListNode<E> node){
		assert (head == null ? tail == null : false) : "Both head and tail must be null, or neither";
		
		if(head == null){
			head = tail = node;
			node.setNext(null);
			node.setPrevious(null);
		} else {
			node.addBefore(head);
			head = node;
		}
	}
	public void removeFirst(){
		if(head != null){
			NLinkedListNode<E> nHead = head.getNext();
			head.remove();
			this.head = nHead;
		}
	}
	
	public void addLast(E value){
		this.addLast(new NLinkedListNode<E>(value));
	}
	public void addLast(NLinkedListNode<E> node){
		assert (head == null ? tail == null : false) : "Both head and tail must be null, or neither";
		
		if(tail == null){
			head = tail = node;
			node.setNext(null);
			node.setPrevious(null);
		} else {
			node.addAfter(tail);
			tail = node;
		}
	}
	public void removeLast(){
		if(tail != null){
			NLinkedListNode<E> nTail = tail.getPrevious();
			tail.remove();
			this.tail = nTail;
		}
	}
	
	public boolean isEmpty(){
		return this.head == null;
	}
	
	public void clear(){
		while(!this.isEmpty())
			this.removeFirst();
	}
	
	@Override
	public Iterator<E> iterator() {
		return new Iterator<E>() {
			NLinkedListNode<E> current = getFirst();
			boolean removed = false;

			@Override
			public boolean hasNext() {
				return current != null && current.getNext() != null;
			}

			@Override
			public E next() {
				removed = false;
				if(this.current != null)
					this.current = this.current.getNext();
				return this.current != null ? this.current.getValue() : null;
			}

			@Override
			public void remove() {
				if(removed || this.current == null)
					return;
				removed = true;
				this.current.remove();
			}
		};
	}

}