class Node():

def __init__(self, data, next):

self.next = next

self.data = data

class Stack():

def __init__(self):

self._top = None

self._size = 0

def push(self, data):

self._top = Node(data, self._top)

self._size += 1

def pop(self):

if self._size == 0:

raise IndexError("pop from empty list!")

data = self._top.data

self._top = self._top.next

self._size -= 1

return data

def items(self):

current = self._top

while current != None:

yield current.data

current = current.next

def size(self):

return self._size