bool hasCycle(struct ListNode *head) {
    if(head == NULL || head->next == NULL)
    {
        return false;
    }
    struct ListNode* slow = head;
    struct ListNode * fast = head->next;
    while(fast->next!= NULL && fast->next->next != NULL)
    {
        if(fast->next == slow->next)
        {
            return true;
        }
        fast = fast->next->next;
        slow = slow->next;
    }
    return false;
}