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 || head->next == NULL){13             return false;14         }15         16         ListNode* fast = head;17         ListNode* slow = head;18         19         while(fast->next != NULL && fast->next->next != NULL)20         {21             fast = fast->next->next;22             slow = slow->next;23             if(fast  == slow){24                 return true;25             }26         }27         return false;28     }29 };