import java.util.NoSuchElementException;

public class TRAI_25_t30_skeleton<E> implements TRAI_25_t30<E> {

    // TODO: use ArrayList _or_ LinkedList as the storage
    // ArrayList<E> data = new ArrayList();
    // LinkedList<E> data = new LinkedList<>();

    /**
     * Number of elements.
     *
     * @return number of elements.
     */
    @Override
    public int size() {
        return 0;
    }

    /**
     * Add element x to stack
     *
     * @param x element to add.
     */
    @Override
    public void push(E x) {
        // TODO
    }

    /**
     * Topmost element.
     *
     * @return topmost element.
     * @throws NoSuchElementException if stack is empty.
     */
    @Override
    public E top() {
        // TODO
        return null;
    }

    /**
     * Second element.
     *
     * @return second element.
     * @throws NoSuchElementException if stack has less than two elements.
     */
    @Override
    public E top2() {

        // TODO
        return null;
    }

    /**
     * Removes topmost element.
     *
     * @return topmost element.
     * @throws NoSuchElementException if stack is empty.
     */
    @Override
    public E pop() {
        // TODO
        return null;
    }

    /**
     * Removes second element.
     *
     * @return second element.
     * @throws NoSuchElementException if stack has less than two elements
     */
    @Override
    public E pop2() {
        // TODO
        return null;
    }
}