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 && fast->next != NULL)15         {16             slow = slow->next;17             //每次fast比slow快一步18             //直到fast == slow19             fast = fast->next->next;20             if (slow == fast)21                 return true;22         }23         return false;24     }25 };