input: Write a python function to find the longest string in a list.
  assert longest_string(['apple', 'bee', 'cat']) == 'apple'
  assert longest_string(['dog', 'elephant', 'ant']) == 'elephant'
  assert longest_string(['a', 'ab', 'abc']) == 'abc'
output: def longest_string(strings):
      return max(strings, key=len)