Solve InterviewCake's stock-price problem

Write a function that returns the maximum profit that a trader could have made
in a day. I solved this using a greedy algorithm which constantly sets the
maximum profit by tracking the lowest price we've encountered.

scratch/deepmind/part_two/stock-price.py

@@ -0,0 +1,54 @@
+import unittest
+
+
+def get_max_profit(xs):
+    if len(xs) < 2:
+        raise Exception('Can only trade with two or more ticker values.')
+    lowest_buy = xs[0]
+    max_profit = None
+    for x in xs[1:]:
+        if not max_profit:
+            max_profit = x - lowest_buy
+        else:
+            max_profit = max(max_profit, x - lowest_buy)
+        lowest_buy = min(lowest_buy, x)
+    return max_profit
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_price_goes_up_then_down(self):
+        actual = get_max_profit([1, 5, 3, 2])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_price_goes_down_then_up(self):
+        actual = get_max_profit([7, 2, 8, 9])
+        expected = 7
+        self.assertEqual(actual, expected)
+
+    def test_price_goes_up_all_day(self):
+        actual = get_max_profit([1, 6, 7, 9])
+        expected = 8
+        self.assertEqual(actual, expected)
+
+    def test_price_goes_down_all_day(self):
+        actual = get_max_profit([9, 7, 4, 1])
+        expected = -2
+        self.assertEqual(actual, expected)
+
+    def test_price_stays_the_same_all_day(self):
+        actual = get_max_profit([1, 1, 1, 1])
+        expected = 0
+        self.assertEqual(actual, expected)
+
+    def test_error_with_empty_prices(self):
+        with self.assertRaises(Exception):
+            get_max_profit([])
+
+    def test_error_with_one_price(self):
+        with self.assertRaises(Exception):
+            get_max_profit([1])
+
+
+unittest.main(verbosity=2)

scratch/deepmind/part_two/todo.org

@@ -10,7 +10,7 @@
 ** DONE Word Cloud Data
 ** DONE Top Scores
 * Greedy Algorithms
-** TODO Apple Stocks
+** DONE Apple Stocks
 ** TODO Highest Product of 3
 ** TODO Product of All Other Numbers
 ** TODO Cafe Order Checker