import javax.xml.soap.Node;
import java.util.Iterator;
public class Stack<T> implements Iterable<T>{
    //记录首届点
    private Node head;
    //栈中元素个数
    private int N;
    public class Node{
        public T item;
        public Node next;
        public Node(T item,Node next){
            this.item=item;
            this.next=next;
        }
    }
    public Stack(){
        this.head=new Node(null,null);
        this.N=0;
    }
    //判断当前栈中的元素个数为0
    public boolean isEmpty(){
        return N==0;
    }
    //获取栈中元素个数
    public int size(){
        return N;
    }
    //把t元素压入栈中
    public void push(T t){
        //找到首届点指向的第一个结点
        Node oldFirst = head.next;
        //创建新节点
        Node newNode = new Node(t, null);
        //让首结点指向新结点
        head.next=newNode;
        //让新结点指向原来的第一个结点
        newNode.next=oldFirst;
        //元素个数+1
        N++;
    }
    public T pop(){
        Node oldFirst = head.next;
        if (oldFirst!=null){
            return null;
        }
        head.next=oldFirst.next;
        N--;
        return oldFirst.item;
    }
    @Override
    public Iterator<T> iterator() {
        return new SIterator();
    }
    private class SIterator implements Iterator{
        private Node n;
        public SIterator(){
            this.n=head;
        }
        @Override
        public boolean hasNext() {
            return n.next!=null;
        }
        @Override
        public Object next() {
            n=n.next;
            return n.item;
        }
    }
}