Linked List Cycle

 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     }