input: Write a python function to find the longest string in a list of words.
  assert longest_string(["apple", "banana", "cherry", "date"]) == "banana"
  assert longest_string(["hello", "world", "python", "programming"]) == "programming"
  assert longest_string(["short", "longer", "longest"]) == "longest"
output: def longest_string(words):
      if not words:
          return ""
      longest = words[0]
      for word in words:
          if len(word) > len(longest):
              longest = word
      return longest