Add solution for challenge #36

Author: Mauricio Klein

Diff for: README.md

@@ -35,3 +35,4 @@
 - [x] [Challenge 33: Merge Overlapping Intervals](challenge-33/)
 - [x] [Challenge 34: Subset sum problem](challenge-34/)
 - [x] [Challenge 35: Count rectangles](challenge-35/)
+- [x] [Challenge 36: Maximum Profit From Stocks](challenge-36/)

Diff for: challenge-36/Makefile

@@ -0,0 +1,4 @@
+test:
+	python test_*.py
+
+.PHONY: test

Diff for: challenge-36/README.md

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+# Maximum Profit From Stocks
+
+This problem was recently asked by Apple.
+
+## Description
+
+You are given an array. Each element represents the price of a stock on that particular day. 
+
+Calculate and return the maximum profit you can make from buying and selling that stock only once.
+
+## Example
+
+```
+Input: (9, 11, 8, 5, 7, 10)
+Output: 5 (Buy on "5" and sell on "10")
+```

Diff for: challenge-36/__init__.py

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+#
+# Time  complexity: O(n)
+# Space complexity: O(1)
+#
+def buy_and_sell(prices):
+  min_price  = float('inf')
+  max_profit = 0
+
+  for price in prices:
+    min_price  = min(min_price , price            )
+    max_profit = max(max_profit, price - min_price)
+
+  return max_profit

Diff for: challenge-36/solver.py

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+import unittest
+from solver import buy_and_sell
+
+class TestSolver(unittest.TestCase):
+  def test_buy_and_sell(self):
+    self.assertEqual(buy_and_sell((9, 11, 8, 5, 7, 10)), 5) # Buy on "5" and sell on "10"
+    self.assertEqual(buy_and_sell((9, 11, 5, 2, 6, 6 )), 4) # Buy on "2" and sell on "6"
+    self.assertEqual(buy_and_sell((4, 4, 4, 4        )), 0) # Buy and sell for the same price
+    self.assertEqual(buy_and_sell((4, 3, 2, 1        )), 0) # Lost money: profit zero
+    self.assertEqual(buy_and_sell((                  )), 0)
+
+if __name__ == "__main__":
+    unittest.main()