下压堆栈的链表实现 java版本

public class LinkedStack<T> {private class Node{// 定义节点的嵌套类T item;Node next;}private Node first;// 栈顶private int N;// 元素数量public boolean isEmpty(){return first == null;}public int size(){return N;}public void push(T newItem){// 向栈顶添加元素Node oldNode = first;first = new Node();first.item = newItem;first.next = oldNode;N++;}public T pop(){// 从栈顶删除元素T popItem = first.item;first = first.next;N--;return popItem;}}