Add solution for challenge #22

Author: Mauricio Klein

README.md

@@ -21,3 +21,4 @@
 - [x] [Challenge 19: First and last indices of an element in a sorted array](challenge-19/)
 - [x] [Challenge 20: Reverse a linked list](challenge-20/)
 - [x] [Challenge 21: Sorting a list with 3 unique numbers](challenge-21/)
+- [x] [Challenge 22: Two-Sum](challenge-22/)

challenge-22/Makefile

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

challenge-22/README.md

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+# Two-Sum
+
+This problem was asked by Facebook.
+
+## Description
+
+You are given a list of numbers, and a target number k.
+
+Return whether or not there are two numbers in the list that add up to k.
+
+## Example
+
+```
+Given [4, 7, 1 , -3, 2] and k = 5, return "true" since 4 + 1 = 5.
+```

challenge-22/__init__.py

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+class Solution():
+  def two_sum(self, nums, target):
+    """
+      Time complexity:  O(n)
+      Space complexity: O(n) (because of the auxiliar set to store the previous values)
+    """
+    previous_nums = set()
+
+    for num in nums:
+      complement = target - num
+
+      if complement in previous_nums:
+        return True
+
+      previous_nums.add(num)
+
+    return False

challenge-22/solver.py

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+import unittest
+from solver import Solution
+
+class TestSolver(unittest.TestCase):
+  def test_solver(self):
+    self.assertEqual(Solution().two_sum([4,7,1,-3,2],  5), True )
+    self.assertEqual(Solution().two_sum([4,7,1,-3,2], 10), False)
+    self.assertEqual(Solution().two_sum([2,2,2,2,2 ],  4), True )
+    self.assertEqual(Solution().two_sum([2         ],  2), False)
+    self.assertEqual(Solution().two_sum([2,4,0     ],  4), True )
+    self.assertEqual(Solution().two_sum([2,4,6,-4  ],  0), True )
+    self.assertEqual(Solution().two_sum([          ],  0), False)
+
+if __name__ == "__main__":
+    unittest.main()