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