input: def concatenate_strings(strings: List[str], separator: str) -> str:
      """Given a list of strings, concatenate them with a specified separator.
      >>> concatenate_strings(["Sample", "Text", "Hello"], "-")
      'Sample-Text-Hello'
      >>> concatenate_strings(["Python", "Code"], " ")
      'Python Code'
      """
output:     result = ""
      for i, s in enumerate(strings):
          result += s
          if i < len(strings) - 1:
              result += separator
      return result