Question

I am trying to reverse a doubly linked list using iterative approach and i’m struck on the part where my original list is being modified even though i’m not using a pointer to pointer style modification of original list.

This is the reverse function i am using, head is a local variable declared in main
like Node* head = NULL; and is already populated with a set of values.

Node* reverse_iterative(Node* head)
{
    if(head == NULL || head->next == NULL)
        return head;

    Node *prev, *current=head, *temp;
    while(current != NULL)
    {
        current->prev = current->next;
        current->next = temp;
        temp = current;
        current = current->prev;
    }
    return temp;
}

This is how i used this in main :

Node* rev = NULL;
rev = reverse_iterative(head);

Here is the output i got:

original list: 15 <=>25 <=>35 <=>45 <=>55 <=>65 <=>75 <=>
making the list actually reverse: 75 <=>65 <=>55 <=>45 <=>35 <=>25 <=>15 <=>
after reversing, the original list now: 15 <=>

I couldn’t get the part where my original head node was being modified.

Answer 1

Look carefully at your while-loop, and consider the following statement on the initial iteration of the loop:

current->next = temp;

What is in temp at the time of this assignment? Exposing more of the code, we have:

Node *prev, *current=head, *temp;
while(current != NULL)
{
    current->prev = current->next;
    current->next = temp;
    temp = current;
    current = current->prev;
}

Note that temp is uninitialized and thus an undefined pointer value. Your original head-ptr next ptr is being reset to garbage held in the uninitialized temp.

Fix this by doing the following:

Node *current=head, *temp=NULL;
while(current != NULL)
{
    temp = current->prev;
    current->prev = current->next;
    current->next = temp;
    temp = current;
    current = current->prev;
}

I’ve not had a chance to test it, but that should get you closer to what you’re looking for.