public class StackOfStrings
StackOfStrings() //create an empty stack
void push(String item)  //insert a new string onto stack
String pop() //remove and return the string most recently added
boolean isEmpty()  //is the stack empty?

//Attention: Stack have only one exit -> only one pointer is enough
/*Corner Cases:
    client add a null item -> IllegalArgumentException
    remove() from empty stack -> NoSuchElementException
*/
public class StackOfStrings {
    private Node first;
    private class Node {
        String item;
        Node next;
    }
    
    public boolean isEmpty() {
        return first == null;
    }
    
    public StackOfStrings {
        Node first = null;
    }

    public void push(String item) {
        //Improve: add exception to deal with invalid operation
        Node oldfirst = first;
        first = new Node(); //Attention: must craete a new instance here
        first.item = item;
        first.next = oldfirst;
    }

    public String pop() {
        String item = first.item;
        first = first.next;
        return item;
    }