This notebook was prepared by Donne Martin. Source and license info is on GitHub.

Solution Notebook¶

Problem: Determine if a string s1 is a rotation of another string s2, by calling (only once) a function is_substring.¶

Constraints¶

Can we assume the string is ASCII?
- Yes
- Note: Unicode strings could require special handling depending on your language
Is this case sensitive?
- Yes
Can we use additional data structures?
- Yes
Can we assume this fits in memory?
- Yes

Test Cases¶

Any strings that differ in size -> False
None, 'foo' -> False (any None results in False)
' ', 'foo' -> False
' ', ' ' -> True
'foobarbaz', 'barbazfoo' -> True

Algorithm¶

Examine the following test case:

s1 = 'barbazfoo'
s2 = 'foobarbaz'

We see that if we can use the given is_substring method if we take compare s2 with s1 + s1:

s2 = 'foobarbaz'
s3 = 'barbazfoobarbazfoo'

Complexity:

Time: O(n)
Space: O(n)

Code¶

In [1]:

class Rotation(object):

    def is_substring(self, s1, s2):
        return s1 in s2

    def is_rotation(self, s1, s2):
        if s1 is None or s2 is None:
            return False
        if len(s1) != len(s2):
            return False
        return self.is_substring(s1, s2 + s2)

Unit Test¶

In [2]:

%%writefile test_rotation.py
import unittest


class TestRotation(unittest.TestCase):

    def test_rotation(self):
        rotation = Rotation()
        self.assertEqual(rotation.is_rotation('o', 'oo'), False)
        self.assertEqual(rotation.is_rotation(None, 'foo'), False)
        self.assertEqual(rotation.is_rotation('', 'foo'), False)
        self.assertEqual(rotation.is_rotation('', ''), True)
        self.assertEqual(rotation.is_rotation('foobarbaz', 'barbazfoo'), True)
        print('Success: test_rotation')


def main():
    test = TestRotation()
    test.test_rotation()


if __name__ == '__main__':
    main()

Overwriting test_rotation.py

In [3]:

%run -i test_rotation.py

Success: test_rotation