input: 
  def find_longest_string(strings: List[str]) -> str:
      """ Given a list of strings, return the longest string.
      >>> find_longest_string(['foo', 'test', 'longstring', 'bar'])
      'longstring'
      >>> find_longest_string(['short', 'longer', 'longest'])
      'longest'
      """
  
output:     longest = ""
      for s in strings:
          if len(s) > len(longest):
              longest = s
      return longest