input: Write a function to calculate the maximum, minimum, and mean number of elements in a given string. The function should take the string as input and return a tuple containing the maximum element, minimum element, and mean value of the elements in the string. If the string is empty, the function should return None for all three values.
  assert get_stats_for_string("abcdef")==('f', 'a', 3.5)
  assert get_stats_for_string("")==(None, None, None)
  assert get_stats_for_string("12345")==('5', '1', 3.0)
output: def get_stats_for_string(s):
      if not s:
          return None, None, None
      elements = list(s)
      max_element = max(elements)
      min_element = min(elements)
      mean_value = sum(elements.index(e) + 1 for e in elements) / len(elements)
      return max_element, min_element, mean_value