c++ - Converting an iterative function to recursive

Question

I know people usually ask this question the other way round, but I have the following problem: I have this iterative function which counts all the nodes in a circular doubly link list containing the data value 20. Now, how do I make this recursive, what will be the base case (terminating case) for the recursive function? Any help is appreciated:

int count(node *start)
{
    int c;
    c = 0;
    if(start == NULL)
        return 0;
    if((start->roll_no) == 20)
        c = 1;
    node *current = start;
    while(current->next != start)
    {
        if((current->next->roll_no) == 20){
        c++;
        }
        current = current->next;
    }

    return c;
}

Answer 1

I think this should work (but note that it requires an extra argument for tracking start):

int count(node *start)
{
    return count_helper(start, start);
}
int count_helper(node *current, node *start)
{
    int c;
    c = 0;
    if(current == NULL)
        return 0;
    if((current->roll_no) == 20)
        c = 1;
    if(current->next == start) return c;
    return (c + count_helper(current->next, start));
}