input: Write a python function to identify the first character that repeats in a string.
  assert first_repeated_char("abcabc") == "a"
  assert first_repeated_char("abc") == "None"
  assert first_repeated_char("123123") == "1"
output: def first_repeated_char(s: str) -> str:
      seen = set()
      for char in s:
          if char in seen:
              return char
          seen.add(char)
      return "None"