# -*- coding: utf-8 -*-

class LinkedList:
    """A representation of a linked list of 
    objects."""
    def __init__(self):
        """Create an empty list."""
        self.first = None
        
    def __len__(self):
        return self.count_nodes(self.first)
        
    def __str__(self):
        return self.show(self.first)
             
     
    def insert_as_first(self,x):  
        """Insert an item to the first position of a list"""
        ny = Node(x)
        ny.next = self.first
        self.first = ny  

    def append(self,x):  
        """Add an item to the en of the list"""
        ny = Node(x)
        current_node=self.first
        if current_node==None:
            self.first=ny
        else:
            while current_node.next!=None:
                current_node=current_node.next
            current_node.next=ny
     
    
    
    def count_nodes(self, node):
        if node==None:
            return 0
        else: 
            nb=self.count_nodes(node.next)
            return 1+nb   
    
    def pop(self):
        """Remove the first list object."""
        x = self.first.value
        self.first = self.first.next
        return x
    
    def show(self, node):
        if node==None:
            return ''
        else: 
            s=self.show(node.next)
            return str(node.value)+'\n'+s   
class Node:
    """An entry in a linked structure (e.g. Stack)."""
    def __init__(self, x):
        """Create a node linked to the given element."""
        self.value = x
        self.next = None   

"""Classroom exercise 1, example 1."""
class Stack:
    """A representation of a last-in-first-out (LIFO) stack of
    objects."""
    def __init__(self):
        """Create an empty stack."""
        self.top = None
    
    def __str__(self):
        return self.show(self.top)
             
    def show(self, node):
        if node==None:
            return ''
        else: 
            s=self.show(node.next)
            return str(node.value)+'\n'+s
        
    def push(self,x):
        """Push an item onto the top of this stack."""
        ny = Node(x)
        ny.next = self.top
        self.top = ny
    
    def pop(self):
        """Remove the top object from this stack and return it."""
        x = self.top.value
        self.top = self.top.next
        return x
    def isempty(self):
        """Test if this stack is empty."""
        ## The naïve solution (below) is unnecessary:
        # if self.top == None:
        #    return True
        # else:
        #    return False
        return self.top == None