Stack Abstract Data Type ADT Stack creates a new stack that is empty. It needs no parameters and returns an empty stack. pushitem 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. Abstract Class for Stacks. def initself: self.top  1 def lenself: return self.top  1 def strself: result   .joinmapstr, self return 'Top '  result def isemptyself: return self.top  1 abstractmethod def iterself: pass abstractmethod def pushself, value: pass abstractmethod def popself: pass abstractmethod def peekself: pass class ArrayStackAbstractStack: def initself, size10: super.init self.array  None  size def iterself: probe  self.top while True: if probe  1: return yield self.arrayprobe probe  1 def pushself, value: self.top  1 if self.top  lenself.array: self.expand self.arrayself.top  value def popself: if self.isempty: raise IndexErrorStack is empty value  self.arrayself.top self.top  1 return value def peekself: expands size of the array. Time Complexity: On Represents a single stack node. def initself, value: self.value  value self.next  None class LinkedListStackAbstractStack: def initself: super.init self.head  None def iterself: probe  self.head while True: if probe is None: return yield probe.value probe  probe.next def pushself, value: node  StackNodevalue node.next  self.head self.head  node self.top  1 def popself: if self.isempty: raise IndexErrorStack is empty value  self.head.value self.head  self.head.next self.top  1 return value def peekself: if self.isempty: raise IndexErrorStack is empty return self.head.value
from abc import ABCMeta, abstractmethod


class AbstractStack(metaclass=ABCMeta):
    """Abstract Class for Stacks."""
    def __init__(self):
        self._top = -1

    def __len__(self):
        return self._top + 1

    def __str__(self):
        result = " ".join(map(str, self))
        return 'Top-> ' + result

    def is_empty(self):
        return self._top == -1

    @abstractmethod
    def __iter__(self):
        pass

    @abstractmethod
    def push(self, value):
        pass

    @abstractmethod
    def pop(self):
        pass

    @abstractmethod
    def peek(self):
        pass


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.
        """
        super().__init__()
        self._array = [None] * size

    def __iter__(self):
        probe = self._top
        while True:
            if probe == -1:
                return
            yield self._array[probe]
            probe -= 1

    def push(self, value):
        self._top += 1
        if self._top == len(self._array):
            self._expand()
        self._array[self._top] = value

    def pop(self):
        if self.is_empty():
            raise IndexError("Stack is empty")
        value = self._array[self._top]
        self._top -= 1
        return value

    def peek(self):
        """returns the current top element of the stack."""
        if self.is_empty():
            raise IndexError("Stack is empty")
        return self._array[self._top]

    def _expand(self):
        """
         expands size of the array.
         Time Complexity: O(n)
        """
        self._array += [None] * len(self._array)  # double the size of the array


class StackNode:
    """Represents a single stack node."""
    def __init__(self, value):
        self.value = value
        self.next = None


class LinkedListStack(AbstractStack):

    def __init__(self):
        super().__init__()
        self.head = None

    def __iter__(self):
        probe = self.head
        while True:
            if probe is None:
                return
            yield probe.value
            probe = probe.next

    def push(self, value):
        node = StackNode(value)
        node.next = self.head
        self.head = node
        self._top += 1

    def pop(self):
        if self.is_empty():
            raise IndexError("Stack is empty")
        value = self.head.value
        self.head = self.head.next
        self._top -= 1
        return value

    def peek(self):
        if self.is_empty():
            raise IndexError("Stack is empty")
        return self.head.value
