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

Challenge 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

Refer to the Solution Notebook. If you are stuck and need a hint, the solution notebook's algorithm discussion might be a good place to start.

Code

In [ ]:
class Node(object):

    def __init__(self, data):
        # TODO: Implement me
        pass


class Stack(object):

    def __init__(self, top=None):
        # TODO: Implement me
        pass

    def push(self, data):
        # TODO: Implement me
        pass

    def pop(self):
        # TODO: Implement me
        pass

    def peek(self):
        # TODO: Implement me
        pass

    def is_empty(self):
        # TODO: Implement me
        pass

Unit Test

The following unit test is expected to fail until you solve the challenge.

In [ ]:
# %load 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()

Solution Notebook

Review the Solution Notebook for a discussion on algorithms and code solutions.