数据结构----背包

 public class Bag<Item> implements Iterable<Item>
               bag()                             创建一个背包
       void add(Item item)              添加一个元素
 boolean isEmpty()                       背包是否为空
          int size()                              背包中的元素数量
 

public class Bag<Item> implements Iterable<Item> {
    private Node first;
    private int n; 

    private class Node {  //结点类
        private Item item;
        private Node next;
    }
    public Bag() 
    {first = null; n = 0;}

    public boolean isEmpty()  { return first == null; }
    public int size() {   return n;}

    public void add(Item item) { 
        Node oldfirst = first;
        first = new Node();
        first.item = item;
        first.next = oldfirst;
        n++;
    }
    public Iterator<Item> iterator()  {  return new ListIterator();  }

    private class ListIterator implements Iterator<Item> {   //迭代器
        private Node current;
        public ListIterator() {  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;
        }
    }
}