import java.util.*;

public class LinkList {
    private Node begin;

    public LinkList() {
        begin = null;
    }

    public String peek() {
        Node res = Optional.ofNullable(begin).orElseThrow(() -> new NoSuchElementException());
        return res.data;
    }

    public void push(String param) {
        Node n = new Node();
        n.data = param;
        n.next = begin;
        begin = n;
    }

    public String pop() {
        Node temp = Optional.ofNullable(begin).orElseThrow(() -> new NoSuchElementException());
        begin = begin.next;
        return temp.data;
    }

    public static void main(String[] args) {
        LinkList list = new LinkList();
        list.push("A");
        list.push("B");
        list.push("C");

        System.out.println("Peek : " + list.peek());
        System.out.println("Pop 1: " + list.pop());
        System.out.println("Pop 2: " + list.pop());
        System.out.println("Pop 3: " + list.pop());
    }
}