下压堆栈（链表实现）

import java.util.Iterator;
public class Stack<Item>
{
private Node first; //栈顶
private int N; //元素数量
private class Node
{ //定义节点的嵌套类
Item item;
Node next;
}
public boolean isEmpty() { return first == null; } // 或： N == 0
public int size() { return N; }
public void push(Item item)
{ // 向栈顶添加元素
Node oldfirst = first;
first = new Node();
first.item = item;
first.next = oldfirst;
N++;
}
public Item pop()
{ //从栈顶删除元素
Item item = first.item;
first = first.next;
N--;
return item;
}
public Iterator<Item> iterator()
{ return new ListIterator(); }
private class ListIterator implements Iterator<Item>
{
private Node current = first;
public boolean hasNext()
{ return current != null; }
public void remove() { }
public Item next()
{
Item item = current.item;
current = current.next;
return item;
}
}
}