import java.util.NoSuchElementException;

public class LinkedList<E> implements List<E> {

    private static class Node<T> {
        private T value;
        private Node<T> previous;
        private Node<T> next;
        private Node(T value, Node<T> previous, Node<T> next) {
            this.value = value;
            this.previous = previous;
            this.next = next;
        }
    }

    private Node<E> head, tail;
    private int size;

    public void addFirst(E elem) {
        if (elem == null) {
            throw new NullPointerException();
        }
        head = new Node<E>(elem, null, head);
        if (tail == null) {
            tail = head;
        }
        size++;
    }

    public void addLast(E elem) {
        if (elem == null) {
            throw new NullPointerException();
        }
        if (head == null) {
            head = tail = new Node<E>(elem, null, null);
        } else {
            tail.next = new Node<E>(elem, tail, null);
            tail = tail.next;
        }
        size++;
    }

    public void add(E elem, int index) {
        if (elem == null ) {
            throw new NullPointerException();
        }
        if (index < 0 || index > size) {
            throw new IndexOutOfBoundsException(index);
        }
        if (index == 0) {
            head = new Node<E>(elem, null, head);
            if (tail == null) {
                tail = head;
            } else {
                head.next.previous = head;
            }
        } else {
            Node<E> before, after;
            before = head;
            for (int i = 0; i < (index-1); i++) {
                before = before.next;
            }
            after = before.next;
            before.next = new Node<E>(elem, before, after);
            if (before == tail) {
                tail = before.next;
            } else {
                after.previous = before.next;
            }    
        }
        size++;
    }

    public E get(int index) {
        if (index < 0 || index >= size) {
            throw new IndexOutOfBoundsException(Integer.toString(index));
        }
        Node<E> p = head;
        for (int i=0; i<index; i++) {
            p = p.next;
        }
        return p.value;
    }

    public E removeFirst() {
        if (head == null) {
            throw new NoSuchElementException();
        }
        E saved = head.value;
        head = head.next;
        if (head == null) {
            tail = null;
        }
        size--;
        return saved;
    }

    public E removeLast() {
        if (head == null) {
            throw new NoSuchElementException();
        }
        E saved;
        if (head.next == null) {
            saved = head.value;
            head = tail = null;
        } else {
            saved = tail.value;
            tail = tail.previous;
            tail.next = null;
        }
        size--;
        return saved;
    }

    public int size() {
        return size;
    }

    public boolean isEmpty() {
        return head == null;
    }

    @Override
    public String toString() {
        String str = "[";
        Node<E> p = head;
        while (p!=null) {
            if (p != head) {
                str += ", ";
            }
            str += p.value;
            p = p.next;
        }
        str += "]";
        return str;
    }
}