input: Write a function to print the longest string from a list of strings. If the list is empty, print an empty string.
  assert longest_string(['foo', 'test', 'longstring', 'bar'])=='longstring'
  assert longest_string(['short', 'longest', 'me'])=='longest'
  assert longest_string(['a', 'b', 'c'])=='a'
  assert longest_string([])==''
output: def longest_string(strings):
      if not strings:
          return ''
      return max(strings, key=len)