hackerrank string exercise

Chapter 1 Arrays and strings/hackerrank/mars_exploration.py

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+# Algorithms > Strings > Mars Exploration
+# https://www.hackerrank.com/challenges/mars-exploration
+
+import unittest
+import numpy as np
+
+''' O(n), O(n)
+def difference(s):
+    inputs = 'SOS' * int(len(s)/3)
+
+    if s is inputs:
+        return 0
+
+    count = 0
+    for a, b in zip(inputs, s):
+        if a != b:
+            count += 1
+
+    return count
+'''
+
+# O(n/3), O(1)
+def difference(s):
+    count = 0
+    for i in range(0, len(s), 3):
+        if s[i] != 'S':
+            count += 1
+        if s[i+1] != 'O':
+            count += 1
+        if s[i+2] != 'S':
+            count += 1
+    return count
+
+class Test(unittest.TestCase):
+
+    data = [('SOS', 0),
+            ('SOA', 1),
+            ('SOSSASSEDAAA', 6)]
+
+    def test(self):
+        for test_string in self.data:
+            res = difference(test_string[0])
+            self.assertEqual(res, test_string[1])
+
+if __name__ == "__main__":
+    unittest.main()