1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     bool hasCycle(ListNode *head) {
12         if (head == NULL or head->next == NULL) {
13             return false;
14         }
15         ListNode *fast = head, *slow = head;
16         while (fast != NULL and fast->next != NULL) {
17             fast = fast->next;
18             if (fast == slow) {
19                 return true;
20             }
21             fast = fast->next;
22             if (fast == slow) { 
23                 return true;
24             }
25             slow = slow->next;
26         }
27         return false;
28     }
29 };