Extra credit (on both exams)

The problem boils down to detecting a loop in a single-linked list.
This has to be done without modifying the list, 
in constant additional space and linear time.

bool isListCircular(const Rec* begin)
{ 
    if (begin==NULL || begin->next == NULL || begin->next->next) 
        return false;
    const Rec *ptr1=begin->next; //these two pointers == O(1) additional memory
    const Rec *ptr2=ptr1 ->next; // ptr2 starts ahead of ptr1 and moves faster
    while (ptr1!=ptr2)           // this can only happen after ptr1 and ptr2
    {                            //    get inside the loop
        ptr2=ptr2->next;         
        if (ptr1==NULL) return false;
        ptr2=ptr2->next;
        if (ptr1==NULL) return false;

        ptr1=ptr1->next; // no need to compare to NULL because ptr2 is ahead
    }
    return true;
};

   Complexity analysis:
    The while loop takes linear time because the distance between ptr1 and ptr2
    is growing by one at every step at most until both pointers get inside 
    the loop and the distance becomes == the loop size. At that moment, the 
    pointers will be equal.
    The necessary number of steps for this to happen is the sum of the number 
    of elements before the loop (prt1 must walk that distance at minimum) 
    plus, in the worst-case, the number of elements in the loop.