input: Write a function to find the longest string in a list.
  assert longest_string(["javascript", "ruby", "python", "java"]) == "javascript"
  assert longest_string(["apple", "banana", "cherry"]) == "banana"
  assert longest_string(["a", "ab", "abc", "abcd"]) == "abcd"
output: def longest_string(strings):
      return max(strings, key=len)