Reversing a linked list that has a Loop

Question

How can we reverse a linked list if it has a loop (ie if the last node is is linked to a node in the middle) ?

Well, I saw that one of the solutions in here and here to detect a loop in a linked list is to reverse it. My doubt is - how is it possible to reverse a linked list,if you do not know where it ends. How can one even reverse a linked list that has a loop ?

Answer 1

Well, first, you're going to need to define what "reverse" means in this context. Probably, what you need to do is

(1) find the link that makes it cyclic

(2) break that link

(3) then reverse the list somehow.

Doing it efficiently is going to mean finding some efficient way to identify the cycle. But if we assume a stack with an operation to tell if a node is already there, then you can just push the nodes onto a stack, checking until you give a link to a node you've already seen. Then pop the stack and voila you have the list in reverse order.

in pseudocode, you need a stack with an isIn operation

   stack:
     init()
     push(node)
     pop() returns node
     isIn(node) returns Boolean

and do something like

 do
    get next node
    if node isIn stack
    then
       while stack not empty
           pop node
       break
    else
       push node in stack
    fi
 od

Answer 2

I think first you have to define what it means to reverse a linked list with a loop. Let's assume you have a linked list:

1->2->3->4->5-|
      ^       |
      |-------|                

So if normally the list would print: 12345345345 if you keep traversing the cycle let's say the reversed linked list would mean all the arrows have reversed their direction and in reverse it should print 54354312

Now you can do something like this:

prev=head;
curr=head->next;
while !curr->visited && curr->next!=end 
    tmp=prev
    prev=curr
    curr=curr->next
    prev->next=tmp
    curr->visited=True

This is pseudo code and their may be small errors in the logic so don't copy verbatim but it's one basic idea.