import nodeclass LLStack(object): """An implementation of the Stack ADT using a linked list.""" def __init__(self): """Create a new, empty LLStack.""" #Empty LLStack is created with head as None & size as 0. self._head = None self._size = 0 def push(self, item): """Put item on top of the LLStack.""" #The new node is created and the current head is linked to it. #The head is reassigned as the new node & size is increased by 1. temp = node.Node(item) temp.next = self._head self._head = temp self._size += 1 def pop(self): """Return and remove the item on top of the LLStack.""" #If the size is not 0, the current head is stored in a temp variable #and the head is reassigned as the node linked to the old head. Size is #decreased by 1. The data from the temp variable is returned. if self._size != 0: temp_node = self._head aelf._head = self._head.next self._size -= 1 return temp_node.data def peek(self): """Return the item on top of the LLStack without removing it.""" #Data of the head is returned if size is not 0. if self._size != 0: return self._head.data def empty(self): """Whether or not the LLStack is empty.""" #Returns True when the head is None. Otherwise is returns False. return self._head == None def size(self): """Return the number of items in the LLStack.""" #Returns size. return self._size