input: Write a function to find unique characters in a string.
  assert findUniqueChars("This is a test") == "Thae"
  assert findUniqueChars("hello") == "heo"
  assert findUniqueChars("abcdabcd") == ""
  assert findUniqueChars("abacabad") == "cd"
output: def findUniqueChars(s):
      unique_chars = []
      for char in s:
          if s.count(char) == 1:
              unique_chars.append(char)
      return ''.join(unique_chars)