Linked List Cycle

 1 class Solution {
 2 public:
 3     bool hasCycle(ListNode *head) {
 4         if(head == NULL)
 5         return false;
 6         ListNode* slow=head; ListNode* fast=head;
 7         while(fast->next!=NULL && fast->next->next!=NULL){
 8             fast=fast->next->next;
 9             slow=slow->next;
10             if(fast == slow)
11             return true;
12         }
13         return false;
14     }
15 };