Java实现Stack类

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

public class Stack<Item> implements Iterable<Item> {
    private int N;
    private Node<Item> first;
    private static class Node<Item> {
        private Item item;
        private Node<Item> next;
    }
    public Stack() {
        first = null;
        N = 0;
    }
    public boolean isEmpty() {
        return first == null;
    }
    public int size() {
        return N;
    }
    public void push(Item item) {
        Node<Item> oldfirst = first;
        first = new Node<Item>();
        first.item = item;
        first.next = oldfirst;
        N ++;
    }
    public Item pop() {
        if(isEmpty()) throw new NoSuchElementException("Stack underflow");
        Item item = first.item;
        first = first.next;
        N --;
        return item;
    }
    public Item peek() {
        if(isEmpty()) throw new NoSuchElementException("Stack underflow");
        return first.item;
    }
    public String toString() {
        StringBuilder s = new StringBuilder();
        for(Item item : this)
            s.append(item + " ");
        return s.toString();
    }
    public Iterator<Item> iterator() {
        return new ListIterator<Item>(first);
    }
    private class ListIterator<Item> implements Iterator<Item> {
        private Node<Item> current;
        public ListIterator(Node<Item> first) {
            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;
        }
    }
    public static void main(String[] args) {
        Stack<String> s = new Stack<String>();
        Scanner in = new Scanner(System.in);
        while(in.hasNext()) {
            String item = in.next();
            if(!item.equals("-")) s.push(item);
            else if(!s.isEmpty()) System.out.println(s.pop() + " ");
            System.out.println("(" + s.size() + " left on stack)");
        }
    }
}