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

- Can we assume the inputs are valid?
- No

- Can we assume the strings are ASCII?
- Yes

- Is this case sensitive?
- Yes

- Is a substring a contiguous block of chars?
- Yes

- Do we expect an int as a result?
- Yes

- Can we assume this fits memory?
- Yes

- None -> TypeError
- '', k = 3 -> 0
- 'abcabcdefgghiij', k=3 -> 6
- 'abcabcdefgghighij', k=3 -> 7

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 longest_substr(self, string, k):
# TODO: Implement me
pass
```

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

In [ ]:

```
# %load test_longest_substr.py
import unittest
class TestSolution(unittest.TestCase):
def test_longest_substr(self):
solution = Solution()
self.assertRaises(TypeError, solution.longest_substr, None)
self.assertEqual(solution.longest_substr('', k=3), 0)
self.assertEqual(solution.longest_substr('abcabcdefgghiij', k=3), 6)
self.assertEqual(solution.longest_substr('abcabcdefgghighij', k=3), 7)
print('Success: test_longest_substr')
def main():
test = TestSolution()
test.test_longest_substr()
if __name__ == '__main__':
main()
```

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