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

Challenge Notebook

Problem: Implement a queue with enqueue and dequeue methods using a linked list.

Constraints

  • If there is one item in the list, do you expect the first and last pointers to both point to it?
    • Yes
  • If there are no items on the list, do you expect the first and last pointers to be None?
    • Yes
  • If you dequeue on an empty queue, does that return None?
    • Yes
  • Can we assume this fits memory?
    • Yes

Test Cases

Enqueue

  • Enqueue to an empty queue
  • Enqueue to a non-empty queue

Dequeue

  • Dequeue an empty queue -> None
  • Dequeue a queue with one element
  • Dequeue a queue with more than one element

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 Queue(object):

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

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

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

Unit Test

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

In [ ]:
# %load test_queue_list.py
import unittest


class TestQueue(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: Dequeue an empty queue')
        queue = Queue()
        self.assertEqual(queue.dequeue(), None)

        print('Test: Enqueue to an empty queue')
        queue.enqueue(1)

        print('Test: Dequeue a queue with one element')
        self.assertEqual(queue.dequeue(), 1)

        print('Test: Enqueue to a non-empty queue')
        queue.enqueue(2)
        queue.enqueue(3)
        queue.enqueue(4)

        print('Test: Dequeue a queue with more than one element')
        self.assertEqual(queue.dequeue(), 2)
        self.assertEqual(queue.dequeue(), 3)
        self.assertEqual(queue.dequeue(), 4)

        print('Success: test_end_to_end')


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


if __name__ == '__main__':
    main()

Solution Notebook

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