Algs4-1.3.50快速出错的迭代器

1.3.50快速出错的迭代器。修改Stack的迭代器代码，确保一旦用例在迭代器中(通过push()或pop()操作）修改集合数据就抛出一个java.util.ConcurrentModificationException异常。
解答：用一个计数器记录push()和pop()操作的次数。在创建迭代器时，将该值记录到Iterator的一个实例变量中。在每次调用hasNext()和next()之前，检查该值是否发生了变化，如果变化则抛出异常。

import java.util.Iterator;
public class Stack<Item> implements Iterable<Item>
{
    private int N;
    private Node first;
    private int pushPopCount;
   
    private class Node
    {
        Item item;
        Node next;
    }
   
    public Stack()
    {
    }
   
    public Stack(Stack s)
    {
        Node right=new Node();
        Node oldright;
        for(Object i:s)
        {
            oldright=right;
            right=new Node();
            right.item=(Item) i;
            right.next=null;
            if(isEmpty())
               first=right;
            else
               oldright.next=right;
            N++;
         }
    }
    public boolean isEmpty()
    {return 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++;
        pushPopCount++;
    }
   
    public Item pop()
    {
        Item item=first.item;
        first=first.next;
        N--;
        pushPopCount++;
        return item;
    }
   
   
    public Item peek()
    {
        Item item=first.item;
        return item;
    }
   
    public void catenation(Stack s)
    {
        while(!s.isEmpty())
        {
            this.push((Item)s.pop());
        }
    }
       
    public Iterator<Item> iterator()
    {return new ListIterator();}
   
    private class ListIterator implements Iterator<Item>
    {
        private Node current=first;
        private int originatedPushPopCount;
        public ListIterator()
        {
            originatedPushPopCount=pushPopCount;
        }
        public boolean hasNext(){return current!=null;}
        public void remove(){}
        public Item next()
        {
            if(originatedPushPopCount!=pushPopCount)
                throw new java.util.ConcurrentModificationException();
            Item item=current.item;
            current=current.next;
            return item;
        }//end next
    }//end class ListIterator
   
    public static void main(String[] args)
    {
        Stack<String> s1=new Stack<String>();
        In in1=new In(args[0]);
        while(!in1.isEmpty())
        {
            String item=in1.readString();
            s1.push(item);
        }//end while
       
       //
        for(String item:s1)
        {
        StdOut.println(item+" ");
        s1.push("push a element");
        }
     }
}