public class BubbleSwapList<E>
{
    private class Node<E> { 
	public E element; 
	public Node<E> next; 
	public Node<E> previous; 
	public Node(E element) {
	    this.element = element; this.next = null; this.previous = null;
	}
    }
    private Node<E> head;
    public BubbleSwapList() { 
	head = null;
    }
    public void add(E element) {
	Node<E> toAdd = new Node<E>(element); 
	if (head != null) {
	    head.previous=toAdd; 
	    }
	toAdd.next = head;
	head = toAdd; 
	}
    
    private void bubbleSwap(Node<E> a)
    {
	Node<E> aprev=a.previous;
	Node<E> anext=a.next;
	if (aprev!=null)
	{
	    aprev.next=anext;
	    if (anext!=null)
		anext.previous=aprev;
	}
	if (anext!=null)
	{
	    Node<E> anextnext= anext.next;
	    anext.next = a;
	    a.previous = anext;
	    a.next = anextnext;
	    if (anextnext!=null)
	    {
		anextnext.previous=a;
	    }
	    if (a==head)
	    {
		head=anext;
	    }
	}
    }
    
    public String toString()
    {
	StringBuilder sb = new StringBuilder();
	Node<E> n = head;
	while (n!=null)
	{
	    sb.append(n.element);
	    sb.append(" ");
	    n = n.next;
	}
	return sb.toString();
    }
    
    public void swaptest1()
    {
	Node<E> n = this.head.next;
	bubbleSwap(n);
    }
    
    public void swaptesth()
    {
	Node<E> n = this.head.next;
	bubbleSwap(n);
    }
    public static void main(String[] args)
    {
	BubbleSwapList<String> bsl = new BubbleSwapList<>();
	bsl.add("AA");
	bsl.add("BB");
	bsl.add("C");
	bsl.add("D");
	System.out.println("LIST:" + bsl);
	bsl.swaptest1();	
	System.out.println("LIST:" + bsl);
	bsl.swaptest1();	
	System.out.println("LIST:" + bsl);
	bsl.swaptesth();	
	System.out.println("LIST:" + bsl);

    }
}