input: Write a python function to find the length of the longest substring without repeating characters.
  assert longest_unique_substring("ABCBCDDEE") == 4
  assert longest_unique_substring("BBBB") == 1
  assert longest_unique_substring("ABDEFGABEF") == 6
  assert longest_unique_substring("GEEKSFORGEEKS") == 7
output: def longest_unique_substring(s):
      char_index = {}
      longest = 0
      start = 0
      
      for index, char in enumerate(s):
          if char in char_index and char_index[char] >= start:
              start = char_index[char] + 1
          char_index[char] = index
          longest = max(longest, index - start + 1)
      
      return longest