input: Write a function to return the longest string in a given list of strings. If the given list is empty then return an empty string.
  assert longest_string(['foo', 'test', 'longstring', 'bar'])=='longstring'
  assert longest_string([])==''
  assert longest_string(['one', 'three', 'two'])=='three'
  assert longest_string(['short', 'even_longer', 'medium'])=='even_longer'
  assert longest_string(['a', 'b', 'c'])=='a'
output: def longest_string(strings):
      if not strings:
          return ''
      return max(strings, key=len)