class Solution {
public:
    bool hasCycle(ListNode *head) {
        ListNode *t1 = head, *t2 = head;
        do
        {
            if (t2 == NULL || t2->next == NULL)
                return 0;
            t1 = t1->next;
            t2 = t2->next->next;
        } while (t1 != t2);
        return 1;
    }
};