Deque

import java.util.Iterator;

public class MyDeque<Item> implements Iterable<Item>{
    private class Node {
        Node next;
        Node pre;
        Item item;
        Node(Item item) {
            this.item = item;
        }
    }

    private int size = 0;
    private Node head;
    private Node tail;

    public void offerFirst(Item item) {
        Node newNode = new Node(item);
        if (head == null) {
            head = newNode;
            tail = newNode;
        } else {
            newNode.next = head;
            head.pre = newNode;
            head = newNode;
        }
        size++;
    }

    public void offerLast(Item item) {
        Node newNode = new Node(item);
        if (tail == null) {
            head = newNode;
            tail = newNode;
        } else {
            tail.next = newNode;
            newNode.pre = tail;
            tail = newNode;
        }
        size++;

    }

    public Item pollFirst(Item item) {
        Node temp = head;
        if (head == null) {
            throw new NullPointerException();
        } else {
            head = head.next;
            head.pre = null;
        }
        size--;
        return temp.item;
    }

    public Item pollLast(Item item) {
        Node temp = head;
        if (tail == null) {
            throw new NullPointerException();
        } else {
            Node last = tail.pre;
            last.next = null;
            last = tail;
        }
        size--;
        return temp.item;
    }

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

    public int size() {
        return size;
    }

    public Iterator<Item> iterator() {
        return new MyDequeIterator();
    }

    private class MyDequeIterator implements Iterator<Item> {

        Node cur = head;
        int i = size;
        public boolean hasNext() {
            return i != 0;
        }

        public Item next() {
            Node temp = cur;
            cur = cur.next;
            return temp.item;
        }
    }

}