static LinkedListNode findBeginning(LinkedListNode head) {
        LinkedListNode fast = head, slow = head;
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
            if (fast == slow) break;
        }
        if (fast == null || fast.next == null) return null;
        fast = head;
        while (fast != slow) {
            fast = fast.next;
            slow = slow.next;
        }
        return fast;
    }