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import java.util.Iterator;

import edu.princeton.cs.algs4.StdOut;

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

private Node first;

private Node last;

private class Node {

private Item item;

private Node next;

private Node prev;

}

public Deque() {

first = null;

last = null;

}

public boolean isEmpty() {

if (first == null)

return true;

return false;

}

public int size() {

// return the number of items on the deque

if (isEmpty()) return 0;

int count = 1;

Node n = first;

while (n != last) {

count++;

n = n.next;

}

return count;

}

public void addFirst(Item item) {

if (item == null) throw new java.lang.NullPointerException();

Node n = new Node();

n.item = item;

if (first != null) {

n.next = first;

first.prev = n;

n.prev = null;

first = n;

} else {

first = n;

last = n;

n.prev = null;

n.next = null;

}

}

public void addLast(Item item) {

if (item == null) throw new java.lang.NullPointerException();

Node n = new Node();

n.item = item;

if (last != null) {

n.prev = last;

last.next = n;

n.next = null;

last = n;

} else {

first = n;

last = n;

n.prev = null;

n.next = null;

}

}

public Item removeFirst() {

if (first == null) throw new java.util.NoSuchElementException();

Item i = first.item;

first = first.next;

if (first != null) {

first.prev = null;

} else {

last = null;

}

return i;

}

public Item removeLast() {

if (last == null) throw new java.util.NoSuchElementException();

Item i = last.item;

last = last.prev;

if (last != null) {

last.next = null;

} else {

first = null;

}

return i;

}

public Iterator<Item> iterator() {

return new DequeIterator();

}

private class DequeIterator implements Iterator<Item> {

private Node current = first;

public boolean hasNext() {

return current != null;

}

public Item next() {

if (current == null) throw new java.util.NoSuchElementException();

Item item = current.item;

current = current.next;

return item;

}

public void remove() {

throw new java.lang.UnsupportedOperationException();

}

}

public static void main(String[] args) {

Deque<Double> deque = new Deque<Double>();

deque.addFirst(0.6);

StdOut.println(deque.removeFirst());

}

}