快慢指针真的很好用！

/*** Definition for singly-linked list.* struct ListNode {*     int val;*     ListNode *next;*     ListNode(int x) : val(x), next(NULL) {}* };*/
class Solution {
public:bool hasCycle(ListNode *head) {//快慢指针ListNode* fast = head;ListNode* slow = head;while(fast != nullptr && fast->next!= nullptr ){fast = fast->next->next;slow = slow->next;if(fast == slow){return true;}}return false;}
};