input: Write a function to find and return the longest string in a given list of strings. 
  assert longest_string(['foo', 'test', 'longstring', 'bar'])=='longstring'
  assert longest_string(['apple', 'banana', 'cat'])=='banana'
  assert longest_string([''])==''
  assert longest_string(['one', 'two', 'three', 'four'])=='three'
  assert longest_string(['', 'xyz', 'uvwxy'])=='uvwxy'
output: def longest_string(strings):
      return max(strings, key=len)