public class Stack<T>
{
    private Node<T> first;
    
    public Stack(){
        this.first = null;
    }
    public boolean isEmpty(){
        return this.first == null;
    }
    public void push(T val){
        Node<T> added = new Node<T>(val);       
        added.next = this.first;
        this.first = added;
    }
    public T pop (){
        if (this.first == null) return null;
        T ret = this.first.wert;
        this.first = this.first.next;
        return ret;
    }
    public T top (){
        if (this.first == null) return null;
        return this.first.wert;
    }
}