content: /*************************************************************************
   *  Compilation:  javac Queue.java
   *  Execution:    java Queue < input.txt
   *  Data files:   http://algs4.cs.princeton.edu/13stacks/tobe.txt  
   *
   *  A generic queue, implemented using a linked list.
   *
   *  % java Queue < tobe.txt 
   *  to be or not to be (2 left on queue)
   *
   *************************************************************************/
  
  import java.util.Iterator;
  import java.util.NoSuchElementException;
  
  /**
   *  The <tt>Queue</tt> class represents a first-in-first-out (FIFO)
   *  queue of generic items.
   *  It supports the usual <em>enqueue</em> and <em>dequeue</em>
   *  operations, along with methods for peeking at the top item,
   *  testing if the queue is empty, and iterating through
   *  the items in FIFO order.
   *  <p>
   *  All queue operations except iteration are constant time.
   *  <p>
   *  For additional documentation, see <a href="http://algs4.cs.princeton.edu/13stacks">Section 1.3</a> of
   *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
   */
  public class Queue<Item> implements Iterable<Item> {
      private int N;         // number of elements on queue
      private Node first;    // beginning of queue
      private Node last;     // end of queue
  
      // helper linked list class
      private class Node {
          private Item item;
          private Node next;
      }
  
     /**
       * Create an empty queue.
       */
      public Queue() {
          first = null;
          last  = null;
      }
  
     /**
       * Is the queue empty?
       */
      public boolean isEmpty() {
          return first == null;
      }
  
     /**
       * Return the number of items in the queue.
       */
      public int size() {
          return N;     
      }
  
     /**
       * Return the item least recently added to the queue.
       * Throw an exception if the queue is empty.
       */
      public Item peek() {
          if (isEmpty()) throw new RuntimeException("Queue underflow");
          return first.item;
      }
  
     /**
       * Add the item to the queue.
       */
      public void enqueue(Item item) {
          Node x = new Node();
          x.item = item;
          if (isEmpty()) { first = x;     last = x; }
          else           { last.next = x; last = x; }
          N++;
      }
  
     /**
       * Remove and return the item on the queue least recently added.
       * Throw an exception if the queue is empty.
       */
      public Item dequeue() {
          if (isEmpty()) throw new RuntimeException("Queue underflow");
          Item item = first.item;
          first = first.next;
          N--;
          if (isEmpty()) last = null;   // to avoid loitering
          return item;
      }
  
     /**
       * Return string representation.
       */
      public String toString() {
          StringBuilder s = new StringBuilder();
          for (Item item : this)
              s.append(item + " ");
          return s.toString();
      } 
   
  
     /**
       * Return an iterator that iterates over the items on the queue in FIFO order.
       */
      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;
  
          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;
          }
      }
  
  
     /**
       * A test client.
       */
      public static void main(String[] args) {
          Queue<String> q = new Queue<String>();
          while (!StdIn.isEmpty()) {
              String item = StdIn.readString();
              if (!item.equals("-")) q.enqueue(item);
              else if (!q.isEmpty()) StdOut.print(q.dequeue() + " ");
          }
          StdOut.println("(" + q.size() + " left on queue)");
      }
  }
  