Constant Complexity For Reversing of a List

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I am reading this question on stack overflow and it was irritating for me that the most people say: reversing a linked list is possible only in O(n). Okay, this is true for a single linked lists, but not for double linked lists like I will show you with my quick and dirty CarouselList:

The key point is the incrementer/decrementer:

static class Incrementer<T> {

 Node<T> getNext(Node<T> node) {
   return node.next;
 }

 Node<T> getStart(Node<T> head) {
   return head;
 }
}

static class Decrementer<T> extends Incrementer<T> {

 @Override
 Node<T> getNext(Node<T> node) {
   return node.previous;
 }

 @Override
 Node<T> getStart(Node<T> head) {
   return head.next;
 }
}

This way the ‘iteration order’ can be easily switched in O(1). Of course this comes to the cost of a bit more expensive iteration but this is another issue …

If the implementation could use an ArrayList (not a linked list) it would be more simple: return different iterators based on a ‘reverse’ attribute. Then either return an iterator with counter++ or return one with counter– if reverse == true

5 thoughts on “Constant Complexity For Reversing of a List”

  1. >Of course this comes to the cost of a bit more expensive iteration

    that means, you shifted the cost of reversing the list from construction time to iteration time.

    the whole point of any java implementation to reverse a list would be to work within the List interface.
    see Iterators.reverse() in google-collections for an example on how to do it within the java api.

  2. > that means, you shifted the cost of reversing the list from construction time to iteration time.

    not really. it depends on your usecase. I needed a fast reverse algorithm and iterate only a few times over the edges.

    > Iterators.reverse()

    yes. that’s the point. But I only wanted to say in public that setReverse(false) is O(1) and not O(n)🙂

  3. Stating this on a thread titled “Reverse a *singly* linked list”, and getting irritated at people who are right? Fail. Plus a minor irritation for allowing things like “Incrementer incrementer = new Decrementer();”.

    By the way, the method is Iterables.reverse().

  4. @Dimitris yes, you are right. he says “what would be the best logic to reverse a singly linked list, in terms of time complexity?” I recognized the “Is there any other alternate to reverse the linked list?” as more important. Sorry for confusion. My fault.

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