input: Write a python function to remove repeated characters from a string.
  assert removeRepeatedChars("aaabbccddeeff") == "abcdef"
  assert removeRepeatedChars("hello world") == "helo wrd"
  assert removeRepeatedChars("aabbcc") == "abc"
  assert removeRepeatedChars("mississippi") == "misp"
  assert removeRepeatedChars("") == ""
output: def removeRepeatedChars(s):
      result = []
      seen = set()
      for char in s:
          if char not in seen:
              seen.add(char)
              result.append(char)
      return ''.join(result)