interactive-coding-challenges/stacks_queues/stack_min/stack_min_challenge.ipynb

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.

• Constraints
• Test Cases
• Algorithm
• Code
• Unit Test
• Solution Notebook

Constraints

• 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-tempty 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 [ ]:

%run ../stack/stack.py
%load ../stack/stack.py

In [ ]:

import sys


class StackMin(Stack):

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

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

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

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

Unit Test

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

In [ ]:

# %load test_stack_min.py
from nose.tools import assert_equal


class TestStackMin(object):

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

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

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

        print('Success: test_stack_min')


def main():
    test = TestStackMin()
    test.test_stack_min()


if __name__ == '__main__':
    main()

Solution Notebook

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