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(['apple', 'banana', 'cherry'])=='banana'
  assert longest_string(['short', 'a', 'I'])=='short'
  assert longest_string([])==''
output: def longest_string(strings):
      if not strings:
          return ''
      return max(strings, key=len)