input: Write a function to find the longest string in a list of strings. If the list is empty, return an empty string.
  assert longest_string(['foo', 'test', 'longstring', 'bar'])=='longstring'
  assert longest_string([])==''
  assert longest_string(['abc', 'a', 'ab', 'abcd'])=='abcd'
  assert longest_string(['short', 'extra short', 'super long string'])=='super long string'
  assert longest_string(['equal', 'long', 'length'])=='length'
output: def longest_string(strings):
      if not strings:
          return ''
      return max(strings, key=len)