棧的特性：先進後出

public class Stack<E> {

    private ArrayList<E> list = new ArrayList<E>();

    //1、置空棧 
    public void empty()
    {
        list.empty();
    }
    //2、判棧空 
    public boolean isEmpty()
    {
        return list.isEmpty();
    }

    //3、入棧  
    public void push(E element)
    {
        list.add(element);
    }
    //4、出棧  
 

    public E pop()
    {
        E element = null;
        if(isEmpty())
        {
            throw new RuntimeException("棧為空");
        }
        //獲取
        element = list.get(list.size()-1);
        //刪除元素
        list.sub(list.size()-1);

        return element;
    }

    //5、取棧頂 
    public E top
 
()
    {
        E element = null;
        if(isEmpty())
        {
            throw new RuntimeException("棧為空");
        }
        //獲取
        element = list.get(list.size()-1);


        return element;
    }

}