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

Solution Notebook

Problem: Given a list of stock prices, find the max profit from 1 buy and 1 sell.

Constraints

  • Are all prices positive ints?
    • Yes
  • Is the output an int?
    • Yes
  • If profit is negative, do we return the smallest negative loss?
    • Yes
  • If there are less than two prices, what do we return?
    • Exception
  • Can we assume the inputs are valid?
    • No
  • Can we assume this fits memory?
    • Yes

Test Cases

  • None -> TypeError
  • Zero or one price -> ValueError
  • No profit
    • [8, 5, 3, 2, 1] -> -1
  • General case
    • [5, 3, 7, 4, 2, 6, 9] -> 7

Algorithm

We'll use a greedy approach and iterate through the prices once.

  • Loop through the prices
    • Update current profit (price = min_price)
    • Update the min price
    • Update the max profit
  • Return max profit

Complexity:

  • Time: O(n)
  • Space: O(1)

Code

In [1]:
import sys


class Solution(object):

    def find_max_profit(self, prices):
        if prices is None:
            raise TypeError('prices cannot be None')
        if len(prices) < 2:
            raise ValueError('prices must have at least two values')
        min_price = prices.pop(0)
        max_profit = prices[0] - min_price
        for price in prices:
            profit = price - min_price
            min_price = min(price, min_price)
            max_profit = max(profit, max_profit)
        return max_profit

Unit Test

In [2]:
%%writefile test_max_profit.py
import unittest


class TestMaxProfit(unittest.TestCase):

    def test_max_profit(self):
        solution = Solution()
        self.assertRaises(TypeError, solution.find_max_profit, None)
        self.assertRaises(ValueError, solution.find_max_profit, [])
        self.assertEqual(solution.find_max_profit([8, 5, 3, 2, 1]), -1)
        self.assertEqual(solution.find_max_profit([5, 3, 7, 4, 2, 6, 9]), 7)
        print('Success: test_max_profit')


def main():
    test = TestMaxProfit()
    test.test_max_profit()


if __name__ == '__main__':
    main()
Overwriting test_max_profit.py
In [3]:
%run -i test_max_profit.py
Success: test_max_profit