public boolean hasCycle(ListNode head) {
        if (head == null || head.next == null)
            return false;
        ListNode slow = head, fast = head;
        while (fast != null) {
            slow = slow.next;
            fast = fast.next;
            if (fast != null)
                fast = fast.next;
            else {
                return false;
            }
            if (fast == slow)
                return true;
        }
        return false;
    }