public void add(Auto s) {
        size++;
        Node temp = new Node(s,null);

        // empty list
        if (top == null) {
            top = temp;
            return;
        }

        // if s is car object
        if (temp.getData() instanceof Car) {
            if(top.getData() instanceof Truck){
                Node current = top;
                top = temp;
                top.next = current;
            }
            if(top.getData() instanceof Car){
                Node current = top;
                top = temp;
                top.next = current;

                while(temp.next.getData() instanceof Car || temp.next != null){
                        if (temp.next.getData().getBoughtOn().after(temp.getData().getBoughtOn())) {
                            return;
                        }
                        if (temp.getData().getBoughtOn().after(temp.next.getData().getBoughtOn())) {
                            Node temp1 = current.next;
                            current = top;
                            current.next = temp;
                            temp.next = temp1;
                        }
                }
            }
        }