input: Write a function to remove duplicate characters from a string.
  assert remove_duplicates("hello")== "heo"
  assert remove_duplicates("programming")== "progamin"
  assert remove_duplicates("aabbcc")== ""
output: def remove_duplicates(s):
      seen = set()
      result = []
      for char in s:
          if char not in seen:
              seen.add(char)
              result.append(char)
      return ''.join(result)