Segfault while reversing a link list with a give size

Question

I have an assignment to write a reversing function to reverse a linked doubly linked list with up to n blocks. I am first getting the endpoint from size in a forloop, then i send the tartpoint and endpoint to an outside function to reverse them. that outside function successfully reveresed head and tail, but i am segfaulting while tring to reverse a given size. I need some help on what is going wrong?

reverse function;

/**
 * Helper function to reverse a sequence of linked memory inside a List,
 * starting at startPoint and ending at endPoint. You are responsible for
 * updating startPoint and endPoint to point to the new starting and ending
 * points of the rearranged sequence of linked memory in question.
 *
 * @param startPoint A pointer reference to the first node in the sequence
 *  to be reversed. 
 * @param endPoint A pointer reference to the last node in the sequence to
 *  be reversed.
 */
template <class T>
void List<T>::reverse( ListNode * & startPoint, ListNode * & endPoint )
{
    if( (startPoint == NULL) || (endPoint == NULL) || (startPoint == endPoint))
    {    return;     }                                                                
    ListNode * curr = startPoint;
    ListNode * nexter = NULL;
    ListNode * prever = endPoint->next;             
    while( curr != NULL)
    { 
       nexter = curr->next;
       curr->next = prever;
       prever = curr;
       curr = nexter;
      prever->prev = curr;
  }

  // now swap start and end pts
   nexter = startPoint;
   startPoint = endPoint;
   endPoint = nexter;

}

Now the reverse function given sze, it should use the above function;

/**
* Reverses blocks of size n in the current List. You should use your
* reverse( ListNode * &, ListNode * & ) helper function in this method!
*
* @param n The size of the blocks in the List to be reversed.
*/
template <class T>
void List<T>::reverseNth( int n )
 {
 if(n == 0)
   return;

  ListNode * startPoint = head;
  ListNode * endPoint = head;
  ListNode * save = NULL;

  for(int i = 0; i< n; i++)                           // need to get endpoint at n
  {
      endPoint = endPoint->next;

   }    
 reverse(startPoint, endPoint);

}

gdb outputs some wierd stuff, probably given functions working on the image afterwards are failing;

 Program received signal SIGINT, Interrupt.
 0x000000000040dcab in __distance<List<RGBAPixel>::ListIterator> (__first=..., __last=...) at      /class/cs225/llvm/include/c++/v1/iterator:488
 488        for (; __first != __last; ++__first)
(gdb) q
A debugging session is active.

 Inferior 1 [process 31022] will be killed.

Answer 1

I need to put this in a reply since I don't know if it's possible to do formatted code in a comment.

That said, you're working way too hard at this:

ListNode * curr = startPoint;
while( curr != NULL)
{
    ListNode * temp = curr->next;
    curr->next = curr->prev;
    curr->prev = temp;
    curr = temp;
}

should get you where you want to be.