Linked List Cycle

Description:

Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

Code:

 1   bool hasCycle(ListNode *head) {

 2        

 3         if (head == NULL)

 4             return false;

 5         ListNode* p = head;

 6         ListNode* q = head;

 7         

 8         while (p&&q)

 9         {

10             p = p->next;

11             q = q->next;

12             if (p==NULL||q==NULL)

13                 return false ;

14             q = q->next;

15             if (p==q)

16                   return true;

17         }

18        return false;

19     }