# Implementation of the Stack ADT using a singly linked list.
class Stack:
 # Creates an empty stack
 def __init__(self):
 self._top = None
 self._size = 0

 # Returns True if the stack is empty or False otherwise.
 def isEmpty(self):
 return self._top is None

 # Returns the number of items in the stack.
 def __len__(self):
 return self._size

 # Returns the top item on the stack without removing it.
 def peek(self):
 assert not self.isEmpty(), "Cannot peek at an empty stack"
 return self._top.item

 # Removes and returns the top item on the stack.
 def pop(self):
 assert not self.isEmpty(), "Cannot pop from an empty stack"
 node = self._top
 self._top = self._top.next
 self._size -= 1
 return node.item

 # Pushes an item onto the top of the stack.
 def push(self, item):
 self._top = _StackNode(item, self._top)
 self._size += 1

class _StackNode:
 def __init__(self, item, link):
 self.item = item
 self.next = link