interview.py

from typing import Iterable, Optional, Tuple
import unittest


def longest_substring_with_two_characters(s: str) -> Optional[Tuple[int]]:
  """Finds the longest substring in s that contains up to 2 distinct characters."""
  substring_start = 0
  current_index = 0
  other_char = None
  continuous_substring_start = 0

  ret = (0, 0)

  while current_index < len(s):
    current_letter = s[current_index]
    if current_letter != s[substring_start]:
      if other_char is None:
        other_char = current_letter
      if current_letter != other_char:
        ret = max(ret, (current_index - substring_start, substring_start))
        substring_start = continuous_substring_start
        other_char = current_letter
        continuous_substring_start = current_index
    if current_letter != s[continuous_substring_start]:
      continuous_substring_start = current_index
    current_index += 1

  return max(ret, (current_index - substring_start, substring_start))[::-1]


class InterviewTestCase(unittest.TestCase):
  def testLongestSubstringWithTwoCharacters(self):
    self.assertEqual(longest_substring_with_two_characters('aabbcccdd'), (4, 5))
    self.assertEqual(longest_substring_with_two_characters('aaaa'), (0, 4))
    self.assertEqual(longest_substring_with_two_characters(''), (0, 0))
    self.assertEqual(
        longest_substring_with_two_characters('aaabcabbcbccd'), (6, 6))
    self.assertEqual(longest_substring_with_two_characters('abababab'), (0, 8))

if __name__ == '__main__':
  unittest.main()