input: Write a python function to find the longest string in a list of strings.
  assert longest_string(["Pear", "Watermelon"]) == "Watermelon"
  assert longest_string(["Apple", "Banana", "Cherry"]) == "Banana"
  assert longest_string(["Dog", "Cat", "Elephant", "Bee"]) == "Elephant"
output: def longest_string(strings):
      return max(strings, key=len)