input: Write a python function to find the longest string in a list.
  assert longest_string(["my", "cat", "is", "lazy"]) == "lazy"
  assert longest_string(["apple", "banana", "grape"]) == "banana"
  assert longest_string(["one", "three", "two"]) == "three"
output: def longest_string(strings):
      return max(strings, key=len)