gistfile1.txt

import java.util.Iterator;
import java.util.NoSuchElementException;

public class Deque<Item> implements Iterable<Item> {

    private Node first;

    private Node last;

    private int size;

    private class Node {
        Item item;
        Node prev;
        Node next;
    }

    // construct an empty deque
    public Deque() {
        first = null;
        last = null;
        size = 0;
    }

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

    // return the number of items on the deque
    public int size() {
        return size;
    }

    // add the item to the front
    public void addFirst(Item item) {
        if (item == null) {
            throw new IllegalArgumentException();
        }
        Node oldfirst = first;
        first = new Node();
        first.item = item;
        first.next = oldfirst;
        if (isEmpty()) {
            last = first;
        } else {
            first.next.prev = first;
        }
        size++;
    }

    // add the item to the end
    public void addLast(Item item) {
        if (item == null) {
            throw new IllegalArgumentException();
        }
        if (isEmpty()) {
            addFirst(item);
            return;
        }
        Node oldlast = last;
        last = new Node();
        last.item = item;
        last.prev = oldlast;
        last.next = null;
        last.prev.next = last;
        size++;
    }

    // remove and return the item from the front
    public Item removeFirst() {
        if (isEmpty()) {
            throw new NoSuchElementException();
        }
        Item item = first.item;
        if(size == 1) {
            first = null;
            last = null;
        } else {
            first = first.next;
            first.prev = null;
        }
        size--;
        return item;
    }

    // remove and return the item from the end
    public Item removeLast() {
        if (isEmpty()) {
            throw new NoSuchElementException();
        }
        Item item = last.item;
        if(size == 1) {
            first = null;
            last = null;
        } else {
            last = last.prev;
            last.next = null;
        }
        size--;
        return item;
    }

    // return an iterator over items in order from front to end
    public Iterator<Item> iterator() {
        return new DequeIterator();
    }

    private class DequeIterator implements Iterator<Item> {

        private Node current = first;

        @Override
        public boolean hasNext() {
            return current != null;
        }

        @Override
        public Item next() {
            if (!hasNext()) {
                throw new NoSuchElementException();
            }
            Item item = current.item;
            current = current.next;
            return item;
        }

        @Override
        public void remove() {
            throw new UnsupportedOperationException();
        }
    }

}