public class SimpleDeque<T> implements MinimalDeque<T> {
    private static class Node<T> {
        T value;
        Node<T> next;
        Node<T> prev;

        Node(T value) {
            this.value = value;
        }
    }

    private Node<T> head;
    private Node<T> tail;
    private int size;

    public SimpleDeque() {
        head = null;
        tail = null;
        size = 0;
    }

    @Override
    public void addFirst(T element) {
        Node<T> newNode = new Node<>(element);
        if (isEmpty()) {
            head = tail = newNode;
        } else {
            newNode.next = head;
            head.prev = newNode;
            head = newNode;
        }
        size++;
    }

    @Override
    public void addLast(T element) {
        Node<T> newNode = new Node<>(element);
        if (isEmpty()) {
            head = tail = newNode;
        } else {
            newNode.prev = tail;
            tail.next = newNode;
            tail = newNode;
        }
        size++;
    }

    @Override
    public T removeFirst() {
        if (isEmpty()) {
            throw new IllegalStateException("Deque is empty");
        }
        T value = head.value;
        head = head.next;
        if (head != null) {
            head.prev = null;
        } else {
            tail = null;
        }
        size--;
        return value;
    }

    @Override
    public T removeLast() {
        if (isEmpty()) {
            throw new IllegalStateException("Deque is empty");
        }
        T value = tail.value;
        tail = tail.prev;
        if (tail != null) {
            tail.next = null;
        } else {
            head = null;
        }
        size--;
        return value;
    }

    @Override
    public T peekFirst() {
        if (isEmpty()) {
            return null;
        }
        return head.value;
    }

    @Override
    public T peekLast() {
        if (isEmpty()) {
            return null;
        }
        return tail.value;
    }

    @Override
    public boolean isEmpty() {
        return size == 0;
    }

    @Override
    public int size() {
        return size;
    }
}