input: Write a python function to remove duplicate characters from a string.
  assert remove_duplicates("mississippi") == "misp"
  assert remove_duplicates("aabbcc") == "abc"
  assert remove_duplicates("hello") == "helo"
output: def remove_duplicates(s: str) -> str:
      seen = set()
      result = []
      for char in s:
          if char not in seen:
              seen.add(char)
              result.append(char)
      return ''.join(result)