传送门

/**
 * 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) {
        ListNode *fast,*slow;
        fast=slow=head;
        while(fast&&fast->next&&fast->next->next){
            slow=slow->next;
            fast=fast->next->next;
            if(fast==slow)return true;
        }
        return false;
    }
};