Add solution for challenge #49

Author: Mauricio Klein

README.md

@@ -48,3 +48,4 @@
 - [x] [Challenge 46: Sudoku solver](challenge-46/)
 - [x] [Challenge 47: Closest coin](challenge-47/)
 - [x] [Challenge 48: Median of window](challenge-48/)
+- [x] [Challenge 49: Buddy strings](challenge-49/)

challenge-49/Makefile

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

challenge-49/README.md

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+# Buddy strings
+
+This problem was asked by AirBnB.
+
+## Description
+
+Given two strings A and B of lowercase letters, return true if and only if we can swap two letters in A so that the result equals B.
+
+## Example
+```
+Input: A = "ab", B = "ba"
+Output: True
+
+Input: A = "ab", B = "ba"
+Output: False
+
+Input: A = "aa", B = "aa"
+Output: True
+
+Input: A = "aaaaaaabc", B = "aaaaaaacb"
+Output: True
+
+Input: A = "", B = "aa"
+Output: False
+```

challenge-49/__init__.py

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+#
+# Time  complexity: O(n) (where "n" is the length of any of the strings)
+# Space complexity: O(1)
+#
+def buddy_strings(a, b):
+  # Check string length match
+  if len(a) != len(b):
+    return False
+
+  # Check if same string with duplicated characters
+  if a == b and len(a) > len(set(a)):
+    return True
+
+  # Calculate the diff between the strings
+  diff_a, diff_b = [], []
+  for i in range(len(a)):
+    if a[i] != b[i]:
+      diff_a.append(a[i])
+      diff_b.append(b[i])
+
+      if len(diff_a) > 2:
+        return False
+
+  # We expect exactly two diffs to make one swap
+  # So, if the number of diffs is different of two, more or
+  # less than one swap is needed and, thus, "false"
+  if len(diff_a) != 2:
+    return False
+
+  # Check if the swaps match
+  if diff_a[0] != diff_b[1] or diff_a[1] != diff_b[0]:
+    return False
+
+  return True

challenge-49/solver.py

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+import unittest
+from solver import buddy_strings
+
+class TestSolver(unittest.TestCase):
+  def test_buddy_strings(self):
+    self.assertEqual(buddy_strings("ab"       , "ba"       ), True )
+    self.assertEqual(buddy_strings("ab"       , "ab"       ), False)
+    self.assertEqual(buddy_strings("aa"       , "aa"       ), True )
+    self.assertEqual(buddy_strings("aaaaaaabc", "aaaaaaacb"), True )
+    self.assertEqual(buddy_strings(""         , "aa"       ), False)
+
+if __name__ == "__main__":
+    unittest.main()