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

Challenge Notebook

Problem: Implement a queue using two stacks.


  • Do we expect the methods to be enqueue and dequeue?
    • Yes
  • Can we assume we already have a stack class that can be used for this problem?
    • Yes
  • Can we push a None value to the Stack?
    • No
  • Can we assume this fits memory?
    • Yes

Test Cases

  • Enqueue and dequeue on empty stack
  • Enqueue and dequeue on non-empty stack
  • Multiple enqueue in a row
  • Multiple dequeue in a row
  • Enqueue after a dequeue
  • Dequeue after an enqueue


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.


In [ ]:
%run ../stack/
%load ../stack/
In [ ]:
class QueueFromStacks(object):

    def __init__(self):
        # TODO: Implement me

    def shift_stacks(self, source, destination):
        # TODO: Implement me

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

    def dequeue(self):
        # TODO: Implement me

Unit Test

In [ ]:
# %load
import unittest

class TestQueueFromStacks(unittest.TestCase):

    def test_queue_from_stacks(self):
        print('Test: Dequeue on empty stack')
        queue = QueueFromStacks()
        self.assertEqual(queue.dequeue(), None)

        print('Test: Enqueue on empty stack')
        print('Test: Enqueue on non-empty stack')
        print('Test: Multiple enqueue in a row')
        num_items = 3
        for i in range(0, num_items):

        print('Test: Dequeue on non-empty stack')
        print('Test: Dequeue after an enqueue')
        self.assertEqual(queue.dequeue(), 0)

        print('Test: Multiple dequeue in a row')
        self.assertEqual(queue.dequeue(), 1)
        self.assertEqual(queue.dequeue(), 2)

        print('Test: Enqueue after a dequeue')
        self.assertEqual(queue.dequeue(), 5)

        print('Success: test_queue_from_stacks')

def main():
    test = TestQueueFromStacks()

if __name__ == '__main__':

Solution Notebook

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