swap(a,b)
{
	if(b == null)
		return a
	if(b->next == null)
	{
		a->next = null;
		b->next = a;
		return b;
	}
	r = swap(b->next, b->next->next);
	if(r == b->next)
	{
		t = r->next;
		b->next = t
		r->next = b;
		a->next = r;
		return a;
	}
	else
	{
		a->next = r;
		b->next = a;
		return b;
	}
}