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

Challenge Notebook

Problem: Find the single different char between two strings.


  • Can we assume the strings are ASCII?
    • Yes
  • Is case important?
    • The strings are lower case
  • Can we assume the inputs are valid?
    • No, check for None
    • Otherwise, assume there is only a single different char between the two strings
  • Can we assume this fits memory?
    • Yes

Test Cases

  • None input -> TypeError
  • 'ab', 'aab' -> 'a'
  • 'aab', 'ab' -> 'a'
  • 'abcd', 'abcde' -> 'e'
  • 'aaabbcdd', 'abdbacade' -> 'e'


Refer to the Solution Notebook. If you are stuck and need a hint, the solution notebook's algorithm discussion might be a good place to start.


In [ ]:
class Solution(object):

    def find_diff(self, str1, str2):
        # TODO: Implement me

Unit Test

The following unit test is expected to fail until you solve the challenge.

In [ ]:
# %load
import unittest

class TestFindDiff(unittest.TestCase):

    def test_find_diff(self):
        solution = Solution()
        self.assertRaises(TypeError, solution.find_diff, None)
        self.assertEqual(solution.find_diff('ab', 'aab'), 'a')
        self.assertEqual(solution.find_diff('aab', 'ab'), 'a')
        self.assertEqual(solution.find_diff('abcd', 'abcde'), 'e')
        self.assertEqual(solution.find_diff('aaabbcdd', 'abdbacade'), 'e')
        self.assertEqual(solution.find_diff_xor('ab', 'aab'), 'a')
        self.assertEqual(solution.find_diff_xor('aab', 'ab'), 'a')
        self.assertEqual(solution.find_diff_xor('abcd', 'abcde'), 'e')
        self.assertEqual(solution.find_diff_xor('aaabbcdd', 'abdbacade'), 'e')
        print('Success: test_find_diff')

def main():
    test = TestFindDiff()

if __name__ == '__main__':

Solution Notebook

Review the Solution Notebook for a discussion on algorithms and code solutions.