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# Stack Abstract Data Type (ADT)

# Stack() creates a new stack that is empty.

# It needs no parameters and returns an empty stack.

# push(item) adds a new item to the top of the stack.

# It needs the item and returns nothing.

# pop() removes the top item from the stack.

# It needs no parameters and returns the item. The stack is modified.

# peek() returns the top item from the stack but does not remove it.

# It needs no parameters. The stack is not modified.

# isEmpty() tests to see whether the stack is empty.

# It needs no parameters and returns a boolean value.

# size() returns the number of items on the stack.

# It needs no parameters and returns an integer.

class AbstractStack:

def __init__(self):

self.top = 0

def isEmpty(self):

return self.top == 0

def __len__(self):

return self.top

def __str__(self):

result = '------\n'

for element in self:

result += str(element) + '\n'

return result[:-1] + '\n------'

class ArrayStack(AbstractStack):

def __init__(self, size=10):

"""

Initialize python List with size of 10 or user given input.

Python List type is a dynamic array, so we have to restrict its

dynamic nature to make it work like a static array.

"""

AbstractStack.__init__(self)

self.array = [None] * size

def push(self, value):

if self.top == len(self.array):

self.expand()

self.array[self.top] = value

self.top += 1

def pop(self):

if self.isEmpty():

raise IndexError("stack is empty")

value = self.array[self.top - 1]

self.array[self.top - 1] = None

self.top -= 1

return value

def peek(self):

if self.isEmpty():

raise IndexError("stack is empty")

return self.array[self.top]

def expand(self):

"""

expands size of the array.

Time Complexity: O(n)

"""

newArray = [None] * len(self.array) * 2 # double the size of the array

for i, element in enumerate(self.array):

newArray[i] = element

self.array = newArray

def __iter__(self):

probe = self.top - 1

while True:

if probe < 0:

raise StopIteration

yield self.array[probe]

probe -= 1

class StackNode(object):

def __init__(self, value):

self.value = value

self.next = None

class LinkedListStack(AbstractStack):

def __init__(self):

AbstractStack.__init__(self)

self.head = None

def push(self, value):

node = StackNode(value)

node.next = self.head

self.head = node

self.top += 1

def pop(self):

if self.isEmpty():

raise IndexError("stack is empty")

value = self.head.value

self.head = self.head.next

self.top -= 1

return value

def peek(self):

if self.isEmpty():

raise IndexError("stack is empty")

return self.head.value

def __iter__(self):

probe = self.head

while True:

if probe is None:

raise StopIteration

yield probe.value

probe = probe.next