Queue.java

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

public class Queue<Item> implements Iterable<Item> {
    private int n = 0;
    private Node first = null, last = null;

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

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

    public int size() {
        return n;
    }

    public Item pick() {
        if (isEmpty()) throw new NoSuchElementException("Queue underflow");
        return first.item;
    }

    public void enqueue(Item item) {
        Node oldlast = last;
        last = new Node();
        last.item = item;
        if (isEmpty()) first = last;
        else oldlast.next = last;
        n++;
    }

    public Item dequeue() {
        if (isEmpty()) throw new NoSuchElementException("Queue underflow");
        Item item = first.item;
        first = first.next;
        n--;
        return item;
    }

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

    // an iterator, doesn't implement remove() since it's optional
    private class ListIterator implements Iterator<Item> {
        private Node current = first;  // node containing current item

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

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

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

    public static void main(String[] args) {
        Queue<String> queue = new Queue<String>();
        while (!StdIn.isEmpty()) {
            String item = StdIn.readString();
            if (item.equals("-"))
                System.out.print(queue.dequeue() + " ");
            else queue.enqueue(item);
        }
        System.out.println();
    }
}