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

Challenge Notebook

Problem: Implement a stack with push, pop, and min methods running O(1) time.


  • Can we assume this is a stack of ints?
    • Yes
  • Can we assume the input values for push are valid?
    • Yes
  • If we call this function on an empty stack, can we return sys.maxsize?
    • Yes
  • Can we assume we already have a stack class that can be used for this problem?
    • Yes
  • Can we assume this fits memory?
    • Yes

Test Cases

  • Push/pop on empty stack
  • Push/pop on non-empty stack
  • Min on empty stack
  • Min on non-empty stack


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 [ ]:
import sys

class StackMin(Stack):

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

    def minimum(self):
        # TODO: Implement me

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

    def pop(self):
        # TODO: Implement me

Unit Test

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

In [ ]:
# %load
import unittest

class TestStackMin(unittest.TestCase):

    def test_stack_min(self):
        print('Test: Push on empty stack, non-empty stack')
        stack = StackMin()
        self.assertEqual(stack.peek(), 5)
        self.assertEqual(stack.minimum(), 5)
        self.assertEqual(stack.peek(), 1)
        self.assertEqual(stack.minimum(), 1)
        self.assertEqual(stack.peek(), 3)
        self.assertEqual(stack.minimum(), 1)
        self.assertEqual(stack.peek(), 0)
        self.assertEqual(stack.minimum(), 0)

        print('Test: Pop on non-empty stack')
        self.assertEqual(stack.pop(), 0)
        self.assertEqual(stack.minimum(), 1)
        self.assertEqual(stack.pop(), 3)
        self.assertEqual(stack.minimum(), 1)
        self.assertEqual(stack.pop(), 1)
        self.assertEqual(stack.minimum(), 5)
        self.assertEqual(stack.pop(), 5)
        self.assertEqual(stack.minimum(), sys.maxsize)

        print('Test: Pop empty stack')
        self.assertEqual(stack.pop(), None)

        print('Success: test_stack_min')

def main():
    test = TestStackMin()

if __name__ == '__main__':

Solution Notebook

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