Conversion from single to circular linked list

Question

Here is the link to the code I wrote for circular linked list. The code is pasted below also.

typedef struct node
{
    int value;
    struct node *next;
}mynode;

mynode *head, *tail, *temp,*sp,*fp;

void add(int value); 
void iterative_reverse();
void print_list();
void findcycle();

int main()
{
    head=(mynode *)0;
    add(1);
    add(2);
    add(3);
    //print_list();
    findcycle();
    return(0);
}

void add(int value)
{
    temp = (mynode *) malloc(sizeof(struct node));
    temp->value=value;
    temp->next=(mynode *)0;
    if(head==(mynode *)0)
    {
        head=temp;
        tail=temp;
    }
    else
    {
        tail->next=temp;
        tail=temp;
        tail->next=head;
        temp->next=head;
    }
}

void findcycle()
{
    if (head == NULL || head->next == NULL)
        printf("null");
    sp=head;
    fp=head->next;
    while (fp != NULL && fp->next != NULL)
    {
        if ((fp == sp) || (fp->next == sp))
                printf("Cycle");
        sp = sp->next;
        fp = fp->next->next;
    }
    printf("Not a Cycle");
}

void print_list()
{
    for(temp=head; temp!=tail; temp=temp->next)
        printf("[%d]->",(temp->value));
}

I had initially written it for single and then changed few pointers to make it circular. I am doing some mistake in it which I am not able to track and hence getting a Timeout. Please suggest.

Thanks a lot.

Answer 1

This looks wrong:

tail->next=temp;
tail=temp;
tail->next=head;
temp->next=head;

It should be (if you're adding the new node at the end of the list and want it to be a circular list, like I'm assuming here):

tail->next=temp;
temp->next=head;
tail=temp;

It's a minor error, anyway: only a redundant assignment.

The really serious trouble is here:

void findcycle()
{
if (head == NULL || head->next == NULL)
            printf("null");
sp=head;
fp=head->next;
while (fp != NULL && fp->next != NULL)
 {
        if ((fp == sp) || (fp->next == sp))
                printf("Cycle");
        sp = sp->next;
        fp = fp->next->next;
 }
printf("Not a Cycle");
}

First of all, what are you trying to accomplish? It isn't clear, so it isn't easy to suggest you how to correct it; the most obvious bug, anyway, is that if the list actually is a circular one, then the loop will go on forever, as there is no exit condition that can ever happen (no one of the pointers will ever become NULL).