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;
}
}