input: Write a function to find the longest string from a list of strings.
  assert longest_string(['cat', 'dog', 'elephant']) == 'elephant'
  assert longest_string(['apple', 'banana', 'cherry', 'date']) == 'banana'
  assert longest_string(['short', 'medium', 'a much longer string']) == 'a much longer string'
output: def longest_string(strings):
      return max(strings, key=len)