input: Write a python function to find the longest string in a list of strings.
  assert longest_string(["apple", "banana", "cherry", "date"]) == "banana"
  assert longest_string(["short", "longer", "longest", "mid"]) == "longest"
  assert longest_string(["a", "ab", "abc", "abcd"]) == "abcd"
output: def longest_string(strings):
      if not strings:
          return ""
      longest = strings[0]
      for string in strings[1:]:
          if len(string) > len(longest):
              longest = string
      return longest