Question

If I send my reverse function on a list I get the expected output. But if I use my reverseNth function I only get the first thing in my list. ReverseNth reverses the list in sections.
For example if I have a list = <1 2 3 4 5>. Calling reverse() will output <5 4 3 2 1>. Calling reverseNth(2) on the list should give <2 1 4 3 5>.

Relevant Code:

void List<T>::reverse( ListNode * & startPoint, ListNode * & endPoint )
{
    if(startPoint == NULL || startPoint == endPoint)
        return;
    ListNode* stop = endPoint;
    ListNode* temp = startPoint;
    startPoint = endPoint;
    endPoint = temp;
    ListNode* p = startPoint; //create a node and point to head

    while(p != stop)
    {
        temp = p->next;
        p->next = p->prev;
        p->prev = temp;
        p = p->next;
    }
}

ReverseNth code:

void List<T>::reverseNth( int n )
{
    if(head == NULL || head == tail || n == 1 || n == 0)
        return;

    if(n >= length)
    {
        reverse(head,tail);
        return;
    }

    ListNode* tempStart = head;
    ListNode* tempEnd;

    for(int j = 0; j < length; j += n)
    {
        // make the end of the section the beginning of the next
        tempEnd = tempStart;
        // set the end of the section to reverse
        for(int i = 0; i < n-1; i ++)
        {
            // check to make sure that the section doesn't go past the length
            if(j+i == length)
                i = n; 
            else
                tempEnd = tempEnd-> next;
        }

        reverse(tempStart, tempEnd);

        if( j == 0)
            head = tempStart;
        if(tempStart == tail)
        {
            tail = tempEnd;
            return;
        }
        else
            tempStart = tempEnd-> next;
    }
    tail = tempEnd;
}

Answer 1

You’re not using startPoint and endPoint in your reverse function. At present, your reverse function reverses the entire list, leaving the old head->next to point to null (since it’s now the end).

I’m guessing the reverse function used to reverse the whole list, but was then extended to take an arbitrary start/end point (possibly an overload?).