This notebook was prepared by Donne Martin. Source and license info is on GitHub.

Solution Notebook

Problem: Implement a stack with push, pop, peek, and is_empty methods using a linked list.

Constraints

  • If we pop on an empty stack, do we return None?
    • Yes
  • Can we assume this fits memory?
    • Yes

Test Cases

Push

  • Push to empty stack
  • Push to non-empty stack

Pop

  • Pop on empty stack
  • Pop on single element stack
  • Pop on multiple element stack

Peek

  • Peek on empty stack
  • Peek on one or more element stack

Is Empty

  • Is empty on empty stack
  • Is empty on one or more element stack

Algorithm

Push

  • Create new node with value
  • Set node's next to top
  • Set top to node

Complexity:

  • Time: O(1)
  • Space: O(1)

Pop

  • If stack is empty, return None
  • Else
    • Save top's value
    • Set top to top.next
    • Return saved value

Complexity:

  • Time: O(1)
  • Space: O(1)

Peek

  • If stack is empty, return None
  • Else return top's value

Complexity:

  • Time: O(1)
  • Space: O(1)

Is Empty

  • If peek has a value, return False
  • Else return True

Complexity:

  • Time: O(1)
  • Space: O(1)

Code

In [1]:
%%writefile stack.py
class Node(object):

    def __init__(self, data, next=None):
        self.data = data
        self.next = next


class Stack(object):

    def __init__(self, top=None):
        self.top = top

    def push(self, data):
        self.top = Node(data, self.top)

    def pop(self):
        if self.top is None:
            return None
        data = self.top.data
        self.top = self.top.next
        return data

    def peek(self):
        return self.top.data if self.top is not None else None

    def is_empty(self):
        return self.peek() is None
Overwriting stack.py
In [2]:
%run stack.py

Unit Test

In [3]:
%%writefile test_stack.py
import unittest


class TestStack(unittest.TestCase):

    # TODO: It would be better if we had unit tests for each
    # method in addition to the following end-to-end test
    def test_end_to_end(self):
        print('Test: Empty stack')
        stack = Stack()
        self.assertEqual(stack.peek(), None)
        self.assertEqual(stack.pop(), None)

        print('Test: One element')
        top = Node(5)
        stack = Stack(top)
        self.assertEqual(stack.pop(), 5)
        self.assertEqual(stack.peek(), None)

        print('Test: More than one element')
        stack = Stack()
        stack.push(1)
        stack.push(2)
        stack.push(3)
        self.assertEqual(stack.pop(), 3)
        self.assertEqual(stack.peek(), 2)
        self.assertEqual(stack.pop(), 2)
        self.assertEqual(stack.peek(), 1)
        self.assertEqual(stack.is_empty(), False)
        self.assertEqual(stack.pop(), 1)
        self.assertEqual(stack.peek(), None)
        self.assertEqual(stack.is_empty(), True)

        print('Success: test_end_to_end')


def main():
    test = TestStack()
    test.test_end_to_end()


if __name__ == '__main__':
    main()
Overwriting test_stack.py
In [4]:
%run -i test_stack.py
Test: Empty stack
Test: One element
Test: More than one element
Success: test_end_to_end

Pythonic-Code

Source: https://docs.python.org/2/tutorial/datastructures.html#using-lists-as-stacks

5.1.1. Using Lists as Stacks
The list methods make it very easy to use a list as a stack, where the last element added is the first element retrieved (“last-in, first-out”). To add an item to the top of the stack, use append(). To retrieve an item from the top of the stack, use pop() without an explicit index. For example:

>>> stack = [3, 4, 5]
>>> stack.append(6)
>>> stack.append(7)
>>> stack
[3, 4, 5, 6, 7]
>>> stack.pop()
7
>>> stack
[3, 4, 5, 6]
>>> stack.pop()
6
>>> stack.pop()
5
>>> stack
[3, 4]