input: Write a python function that takes a list of strings and returns the longest string. If multiple strings share the maximum length, return the one that appears first. If the list is empty, return None.
  assert find_longest([]) == None
  assert find_longest(['a', 'b', 'c']) == 'a'
  assert find_longest(['a', 'bb', 'cccc', 'ddd', 'eeee', 'xyz']) == 'cccc'
output: def find_longest(strings):
      if not strings:
          return None
      longest = strings[0]
      for string in strings:
          if len(string) > len(longest):
              longest = string
      return longest