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