from abc import ABC, abstractmethod

class ListeElement(ABC):
    @abstractmethod
    def length(self):
        pass


class Element(ListeElement):
    def __init__(self, data):
        self.data = data
        self.next = None

    def length(self):
        # Rekursive Methode zur Berechnung der Länge
        return 1 + self.next.length()


class Abschluss(ListeElement):
    def length(self):
        # Ein Abschluss hat keine Länge
        return 0


class Liste:
    def __init__(self):
        self.head = Abschluss()  # Der Kopf der Liste ist ein Abschluss

    def insert(self, value):
        new_element = Element(value)
        new_element.next = self.head
        self.head = new_element

    def remove_first(self):
        if isinstance(self.head, Abschluss):
            return  # Liste ist leer
        self.head = self.head.next

    def length(self):
        return self.head.length()

    def show_list(self):
        current = self.head
        while not isinstance(current, Abschluss):
            print(current.data)
            current = current.next

    def search(self, value):
        current = self.head
        while not isinstance(current, Abschluss):
            if current.data == value:
                return current
            current = current.next
        return None

    def insert_before(self, value, comp_value):
        new_element = Element(value)
        if isinstance(self.head, Abschluss):
            return  # Liste ist leer

        if self.head.data == comp_value:
            new_element.next = self.head
            self.head = new_element
            return

        current = self.head
        while current.next is not None and current.next.data != comp_value:
            current = current.next

        if current.next is not None:
            new_element.next = current.next
            current.next = new_element