public String value;
    public AtomicReference<Node> next;
**************************************
    Node head;
    final Node tail;
    //@ -> O -> O -> ... -> @ -<
    public void push(String x) {
        Node newNode = new Node(x, head.next.get());
        while(!head.next.compareAndSet(newNode.next.get(), newNode)) {
            newNode.next.set(head.next.get());
        }
    }