class Stack:
  def __init__(self):
    self.items = []
    self.max = 5
    
  def push(self, item):
    if self.items.len < 5:
      self.items.append(item)
    else:
      print('abort push in order to prevent stack overflow')
      
  def pop(self):
    if not self.items:
      self.items.pop()
    else:
      print('stack is empty. abort pop to prevent stack underflow')
      
  def is_empty(self):
    return not self.items
  
  def peek(self):
    return self.items[len(self.items)-1]

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

class Stack:
  def __init__(self):
    self.head = None
    
  def is_empty(self):
    return not self.head
  
  def push(self, item):
    node_data = Node(item)
    node_data.next = self.head
    self.head = node_data
 
  def pop(self):
    if self.is_empty():
      return 
    ret_data = self.head.data
    self.head = self.head.next
    return ret_data
 
  def peek(self):
    if self.is_empty():
      return
    return self.head.data