"""

Stack()

@description: creates a new empty stack, it needs no parameters

@params: no parameters needed

@return: an empty stack

push(item)

@description: appends item to the top of the stack, the stack is modified

@params: the item

@return: nothing

pop()

@description: removes the top item from the stack, the stack is modified

@params: no parameters needed

@return: the item

peek() / top()

@description: gets the top item from the stack

@params: no parameters needed

@return: the top item from the stack

size()

@description: gets the number of items on the stack

@params: no parameters needed

@return: the size of the stack

isEmpty()

@description: tests to see whether the stack is empty

@params: no parameters needed

@return: boolean

"""

class Node:

def __init__(self, data):

self.data = data

self.next = None

class LinkedStack:

def __init__(self):

self.head = None

self.size = 0

def isEmpty(self):

if self.head == None:

return True

return False

def size(self):

return self.size

def push(self, data):

if self.head == None:

self.head = Node(data)

new_node = Node(data)

new_node.next = self.head

self.head = new_node

def pop(self):

if self.isEmpty():

raise Exception("Popping from an empty stack")

popped_node = self.head

self.head = self.head.next

popped_node.next = None

return popped_node.data

def peek(self):

if self.isEmpty():

raise Exception("Peeking from an empty stack")

return self.head.data

def testLinkedStack():

s = LinkedStack()

assert s.isEmpty() == True

s.push(4)

s.push(5)

assert s.isEmpty() == False

assert s.peek() == 5

s.pop()

assert s.peek() == 4

if __name__ == "__main__":

testLinkedStack()

print('ALL TEST CASES PASSED')