下压堆栈Stack

下压堆栈，链表实现:

1. 可以处理任意类型的数据
2. 所需的空间总是和集合的大小成正比
3. 操作所需的时间总是和集合的大小无关

package com.vadonmo.exp.example;import java.util.Iterator;/** * 下压堆栈，链表实现 *  * @author vadon * */public class Stack<Item> implements Iterable<Item> {    // 栈顶    private Node first;    // 元素数量    private int N;    // 节点定义    private class Node {        Item item;        Node next;    }    public boolean isEmpty() {        return first == null;// or N == 0    }    public int size() {        return N;    }    public void push(Item item) {        Node oldfirst = first;        first = new Node();        first.item = item;        first.next = oldfirst;        N++;    }    public Item pop() {        Item item = first.item;        first = first.next;        N--;        return item;    }    @Override    public Iterator<Item> iterator() {        return new ListIterator();    }    private class ListIterator implements Iterator<Item> {        private Node current = first;        @Override        public boolean hasNext() {            return current != null;        }        @Override        public Item next() {            Item item = current.item;            current = current.next;            return item;        }        @Override        public void remove() {        }    }}