数据结构----栈

 public class Stack<Item> implemrnts Iterable<Item>
               Stack()                              创建一个空栈
       void push(Item item)                添加一个元素
       Item pop()                                删除最近添加的元素
 boolean isEmpty()                          栈是否为空
          int size()                                栈中元素的数量
 

public class Stack<Item> implements Iterable<Item> {    //实现迭代器接口
    private Node<Item> first;   //头指针 
    private int n;      //栈中元素数量        
	//节点类
    private static class Node<Item> {
        private Item item;
        private Node<Item> next;
    }
	//构造器
    public Stack() {first = null;n = 0;}
	//是否为空
    public boolean isEmpty() {return first == null;}
	//当前元素数量    
    public int size() {return n;}
	//压栈
    public void push(Item item) {
        Node<Item> oldfirst = first;
        first = new Node<Item>();
        first.item = item;
        first.next = oldfirst;
        n++;
    }
	//出栈
    public Item pop() {
        if (isEmpty()) throw new NoSuchElementException("Stack underflow");
        Item item = first.item;
        first = first.next;            
        n--;
        return item;                   
    }

    //新建迭代器
    public Iterator<Item> iterator() {
        return new ListIterator<Item>(first);
    }
	//实现迭代器
    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;
        }
    }
}