miguel soln to string rotation 1.9

Author: miguelHx

Python/chapter01/1.9 - String Rotation/miguel_1.9_sol.py

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+"""
+Python version 3.7.0
+1.9 - String Rotation
+Assume you have a method isSubstring which
+checks if one word is a substring
+of another.  Given two strings, s1 and s2,
+write code to check if s2 is a
+rotation of s1 using only one call to isSubstring
+(e.g. 'waterbottle' is a rotation of 'erbottlewat')
+"""
+import unittest
+
+
+def is_substring(sub: str, s: str) -> bool:
+    """
+    is_substring checks if 'sub' is a substring of 's'
+    :param sub: the substring
+    :param s: the alleged full string
+    :return: true if sub is a substring of s, false otherwise
+    """
+    return sub in s
+
+
+def string_rotation(s1: str, s2: str) -> bool:
+    """
+    Given two strings, string_rotation will check if s2 is a rotation of s1
+    using only one call to isSubstring.
+    We will loop through the original string and compare chars of
+    rotated string.  At the first character equality, we will
+    compare the rest of the original string to the rotated
+    string's substring of the same length.
+    If they match, then we save the index of the
+    rotated string + 1, and if whatever is left of
+    s1 is a substring of s2, then we must have a
+    rotation since we compared the complementary part already.
+    Runtime: Worst case:  O(n^2)
+    Space Complexity:  O(1)
+    :param s1: the rotated string
+    :param s2: the original string
+    :return: true if s2 is a rotation of s1, false otherwise
+    """
+    # not rotation if lengths don't match
+    if len(s1) != len(s2):
+        return False
+    # if they match, but both empty, return true
+    if s1 == '' and s2 == '':
+        return True
+    size = len(s1)
+    start_idx = -1
+    for i, c in enumerate(s2):
+        if c != s1[0]:
+            continue
+        # check if substring in s2 from i to the end
+        # matches s1 from beginning to remaining length of s2
+        if s2[i:] == s1[0:size - i]:
+            # store starting idx of remaining substring in s2
+            start_idx = size - i
+            break
+    if start_idx == -1:
+        return False
+    if is_substring(s1[start_idx:], s2):
+        return True
+    return False
+
+
+class TestStringRotation(unittest.TestCase):
+
+    def setUp(self):
+        self.str_rotation_cases = [
+            ('erbottlewat', 'waterbottle', True),
+            ('erbottlewet', 'weterbottle', True),
+            ('same', 'same', True),
+            ('oneoff', 'oneof', False),
+            ('erbottleabc', 'waterbottle', False),
+            ('ion?rotat', 'rotation?', True),
+            ('', '', True),
+            ('a', 'a', True),
+            ('a', 'b', False),
+            ('erbottlewat', 'eeeeeeeeee', False)
+        ]
+
+        self.is_substring_cases = [
+            ('swag', 'swagger', True),
+            ('swagger', 'swag', False),
+            ('test', 'testing', True),
+            ('', 'sdfkjslf', True),
+            ('abcd', 'defg', False)
+        ]
+
+    def test_is_substring(self):
+        for sub, s, expected in self.is_substring_cases:
+            self.assertEqual(is_substring(sub, s), expected, msg=(sub, s, expected))
+
+    def test_string_rotation(self):
+        for s1, s2, expected in self.str_rotation_cases:
+            self.assertEqual(string_rotation(s1, s2), expected, msg=(s1, s2, expected))
+
+
+if __name__ == '__main__':
+    unittest.main()