This notebook was prepared by [Donne Martin](http://donnemartin.com). Source and license info is on [GitHub](https://github.com/donnemartin/interactive-coding-challenges).

Solution Notebook

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

Constraints
Test Cases
Algorithm
Code
Unit Test
Solution Notebook

Constraints

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

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


def is_rotation(s1, s2):
    if s1 is None or s2 is None:
        return False
    if len(s1) != len(s2):
        return False
    s3 = s1 + s1
    return is_substring(s2, s3)

Unit Test



In [2]:

    
%%writefile test_rotation.py
from nose.tools import assert_equal


class TestRotation(object):

    def test_rotation(self):
        assert_equal(is_rotation('o', 'oo'), False)
        assert_equal(is_rotation(None, 'foo'), False)
        assert_equal(is_rotation('', 'foo'), False)
        assert_equal(is_rotation('', ''), True)
        assert_equal(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