This notebook was prepared by [Donne Martin](http://donnemartin.com). Source and license info is on [GitHub](https://github.com/donnemartin/interactive-coding-challenges).

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
If we call this function on an empty stack, can we return maxsize?
- Yes
Can we assume we already have a stack class that can be used for this problem?
- Yes

Test Cases

Push/pop on empty stack
Push/pop on non-empty 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 MyStack(Stack):

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

    def min(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 = MyStack()
        stack.push(5)
        assert_equal(stack.peek(), 5)
        assert_equal(stack.min(), 5)
        stack.push(1)
        assert_equal(stack.peek(), 1)
        assert_equal(stack.min(), 1)
        stack.push(3)
        assert_equal(stack.peek(), 3)
        assert_equal(stack.min(), 1)
        stack.push(0)
        assert_equal(stack.peek(), 0)
        assert_equal(stack.min(), 0)

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