算法Sedgewick第四版-第1章基础-015一stack只保留last指针

 /*************************************************************************
  *
  *  A generic queue, implemented using a *circular* linked list.
  *  (Exercise 1.3.29)
  *
  *  % java Ex_1_3_29 < tobe.txt
  *  to be or not to be (2 left on queue)
  *
  *************************************************************************/
 
 import java.util.Iterator;
 import java.util.NoSuchElementException;
 
 public class Ex_1_3_29<Item> implements Iterable<Item> {
     private int N;
     private Node last;
 
     private class Node {
         private Item item;
         private Node next;
     }
 
    /**
      * Create an empty queue.
      */
     public Ex_1_3_29() {
         last  = null;
     }
 
    /**
      * Is the queue empty?
      */
     public boolean isEmpty() {
         return last == 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 last.next.item;
     }
 
    /**
      * Add the item to the queue.
      */
     public void enqueue(Item item) {
         Node x = new Node();
         x.item = item;
         if (isEmpty())
             x.next = x;
         else
         {
             x.next = last.next;
             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 = last.next.item;
         if (last.next == last)
             last = null;
         else
             last.next = last.next.next;
         N--;
         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();
     }
 
     private class ListIterator implements Iterator<Item> {
         private int n = N;
         private Node current = last;
 
         public boolean hasNext()  { return n > 0; }
         public void remove()      { throw new UnsupportedOperationException();  }
 
         public Item next() {
             if (!hasNext()) throw new NoSuchElementException();
             Item item = current.next.item;
             current = current.next;
             n--;
             return item;
         }
     }
 
     public static void main(String[] args) {
         Ex_1_3_29<String> q = new Ex_1_3_29<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: [ " + q + "])");
     }
 }