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