Reversing Portions of a Doubly Linked Linked (c++)

Question

I'm attempting to right a method which reverses a doubly linked list.

template <class T>
void List<T>::reverse() {
  reverse(head_, tail_);
}

template <class T>
void List<T>::reverse(ListNode*& startPoint, ListNode*& endPoint) {
  if(startPoint == NULL || endPoint == NULL) return;

  ListNode* currPoint = *&startPoint;
  ListNode* end = endPoint->next;

  while(currPoint != end ) {
    ListNode* tmp = currPoint->next;
    currPoint->next = currPoint->prev;
    currPoint->prev = tmp;

    if(tmp == end) {
      endPoint = startPoint;
      startPoint = currPoint;
    }

    currPoint = tmp;
  }
}

So, head_ and tail_ are the pointers to the beginning and end of the DLL. The actual process of reversing the list should be fairly simple - reverse the prev and next pointer for every ListNode in the sequence.

As you can see, I'm attempting to make it so that the second method can reverse any sub-part of the DLL. I'll be using the second method in other methods, but for now my only goal is to make it work for reversing the entire list. I think that the biggest issue is that head_ isn't being updated appropriately, since nothing is present in the when I print the list.

When I print my fairly basic test it simply shows:

Expected:   < 9 8 7 6 5 4 3 2 1 0 >
Actual:     < >

One implementation was showing the "9" in the actual output, but I'm fairly certain that was because the first portion of the list was simply being thrown away.

Answer 1

You can simplify the problem into three steps :

1. Store prev of startpoint and next of endpoint, and make them point to nullptr.
2. Reverse the sublist using the same approach which is used for a full length of linked list.
3. Connect start and end nodes of reversed sublist with the stored values in step 1.

This way

[prev] startpoint  ... endpoint [nxt]

[prev]   (nullptr) startpoint... endpoint (nullptr)   [nxt]

[prev]   (nullptr) reverse(startpoint...endpoint) (nullptr)   [nxt]

[prev] begin(reversed-sublist) ...end(reversed-sublist)  [nxt]