input: Write a function to find and return the longest string in a given list of strings. If the list is empty, return an empty string.
  assert longest_string(['foo', 'test', 'longstring', 'bar'])=='longstring'
  assert longest_string(['a', 'bb', 'ccc', 'dddd'])=='dddd'
  assert longest_string([])==''
  assert longest_string(['short', 'long', 'longer'])=='longer'
output: def longest_string(strings):
      if not strings:
          return ''
      return max(strings, key=len)