input: from typing import List
  
  
  def roman_to_arabic(roman: str) -> int:
      """Converts a given Roman numeral string to its Arabic (integer) form.
  
      >>> roman_to_arabic("XV")
      15
      >>> roman_to_arabic("MDCLXVI")
      1666
      """
output:     roman_dict = {'I': 1, 'V': 5, 'X': 10, 'L': 50, 'C': 100, 'D': 500, 'M': 1000}
      arabic = 0
      prev_value = 0
  
      for char in reversed(roman):
          value = roman_dict[char]
          if value < prev_value:
              arabic -= value
          else:
              arabic += value
          prev_value = value
  
      return arabic