input: Write a function to find the longest string in a list of strings.
  assert find_longest_string(['Python', 'is', 'a', 'programming', 'language']) == 'programming'
  assert find_longest_string(['short', 'longer', 'longest']) == 'longest'
  assert find_longest_string(['one', 'two', 'three', 'four']) == 'three'
output: def find_longest_string(strings):
      if not strings:
          return None
      return max(strings, key=len)