class Solution {
public:
    bool hasCycle(ListNode *head) 
    {
        if (!head)
        {
            return false;
        }
        ListNode *pF = head;
        ListNode *pS = head;
        while (pS->next)
        {
            pS = pS->next;
            if (pS == pF)
            {
                return true;
            }
            pS = pS->next;
            if (!pS)
            {
                return false;
            }
            pF = pF->next;
        }
        return false;
    }
};