Loop detection in a Linked list

Question

Consider the following code which is used to detect the loop in the linked list. Find the missing statement A?

p = head;
q = head->next;
while(A)
{
    if(p == q) exit(0) //loop detected
    p = p->next;
    q = (q->next)?(q->next->next) : q->next;
}

Which of the following are correct statements

a) p!=NULL

b) q!=NULL

c) (p!=NULL) && (q!=NULL)

d) (p!=NULL) || (q!=NULL)

Given ans is c.......but I think b could also be correct if not than why..... Please explain

Comments

here

if(p==null)

return;is not given right so we have to check that also.

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q = head->next;

before this assignment, we should have a NULL check for p itself. I think here assumption is at least head node is present in the list. so p will be head and q will be NULL before starting of while loop. in that case, Null check for q alone would be sufficient. 

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@Anu007

Actually before the null check for p inside loop, a crash can occur while assigning q=p->next in case p is null. this is not a correct code. 

Answer 1

Actually , here p and q  pointer is incrementing 1:3 manner. As p is incrementing very less than q , so, loop will exists when q coming back and address of q pointer will match with p pointer.

So, we can tell if there is no loop, q will go end of list first and will accept

q==NULL

and will terminate

Otherwise p and q both get a valid pointer value and keep moving.

So, only checking of q pointer will be sufficient and enough

Ans will be B)