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