input: Write a function called longest_string that takes a list of strings as input and returns the longest string in the list. If the list is empty, return an empty string.
  assert longest_string(['foo', 'test', 'longstring', 'bar'])=='longstring'
  assert longest_string(['short', 'tiny', 'tiny', 'verylongstring'])=='verylongstring'
  assert longest_string(['a', 'b', 'c', 'd'])=='a'
  assert longest_string([])==''
output: def longest_string(strings):
      if not strings:
          return ''
      return max(strings, key=len)