public static DoublyLinkedListNode reverse(DoublyLinkedListNode llist) {
        if(llist == null || llist.next == null){
            return llist;
        }
        DoublyLinkedListNode last = null;
        DoublyLinkedListNode curr = llist;
        
        while(curr!=null){
            last = curr.prev;
            curr.prev = curr.next;
            curr.next = last;
            curr = curr.prev;
        }
        return last.prev;
    }

}