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         ListNode *fast = head;
13         ListNode *slow = head;
14         while ((fast != NULL) && (slow != NULL) && (fast->next != NULL)) {
15             fast = fast->next->next;
16             slow = slow->next;
17             if (fast == slow) {
18                 return true;
19             }
20         }
21         return false;
22     }
23 };