input: Write a python function to find the longest string in a list of strings.
  assert longest_string(['foo', 'test', 'longstring', 'bar']) == 'longstring'
  assert longest_string(['one', 'three', 'twenty', 'ten']) == 'twenty'
  assert longest_string(['apple', 'banana', 'pear']) == 'banana'
output: def longest_string(strings):
      if not strings:
          return None
      return max(strings, key=len)