cs_lb2

class Node:
    def __init__(self, data, next=None):
        self.__data = data
        self.__next__ = next

    def get_data(self):
        return self.__data

    def change_data(self, new_data):
        self.__data = new_data

    def __str__(self):
        return f'data: {self.__data}, next: {self.__next__.__data if self.__next__ is not None else None}'


class LinkedList:
    def __init__(self, head=None):
        self.__head__ = head
        self.__length = 0

    def __len__(self):
        length = 0
        if self.__head__ is not None:
            length += 1
            tmp = self.__head__
            while tmp.__next__ is not None:
                length += 1
                tmp = tmp.__next__
            return length
        else:
            return 0

    def append(self, element):
        self.__length += 1
        elem = Node(element, None)
        if self.__head__ is not None:
            tmp = self.__head__
            while tmp.__next__ is not None:
                tmp = tmp.__next__
            tmp.__next__ = elem
        else:
            self.__head__ = elem

    def __str__(self):
        if self.__head__ is not None:
            tmp = self.__head__
            elems = f'{tmp}'
            while tmp.__next__ is not None:
                tmp = tmp.__next__
                elems += "; " + tmp.__str__()
            res = f'LinkedList[length = {self.__length}, [{elems}]]'
        else:
            res = f'LinkedList[]'
        return res

    def pop(self):
        if self.__head__ is None:
            raise IndexError("LinkedList is empty!")
        else:
            tmp1 = self.__head__
            tmp2 = tmp1.__next__
            while tmp2 is not None and tmp2.__next__ is not None and tmp2:
                tmp1 = tmp2
                tmp2 = tmp2.__next__
            if tmp1 == self.__head__ and self.__length == 1:
                self.__head__ = None
            else:
                tmp1.__next__ = None
            self.__length -= 1

    def change_on_end(self, n, new_data):
        if self.__length < n or n <= 0:
            raise KeyError(f"<element> doesn't exist!")
        else:
            tmp = self.__head__
            length = self.__length
            while length != n:
                length -= 1
                tmp = tmp.__next__
            tmp.change_data(new_data)

    def clear(self):
        self.__head__ = None
        self.__length = 0