class Node:

def __init__(self,value):

self.value = value

self.next = None

class Stack:

def __init__(self):

self.head = None

self.num_elements = 0

def push(self, value):

new_node = Node(value)

# if stack is empty

if self.head is None:

self.head = new_node

else:

new_node.next = self.head # place the new node at the head (top) of the linked list

self.head = new_node

self.num_elements += 1

def pop(self):

if self.is_empty():

return

value = self.head.value # copy data to a local variable

self.head = self.head.next # move head pointer to point to next node (top is removed by doing so)

self.num_elements -= 1

return value

def size(self):

return self.num_elements

def is_empty(self):

return self.num_elements == 0