input: Write a python function to find the longest string in a list.
  assert longest_string(["apple", "banana", "cherry", "date"]) == "banana"
  assert longest_string(["short", "longer", "longest"]) == "longest"
  assert longest_string(["a", "abc", "ab"]) == "abc"
output: def longest_string(strings):
      if not strings:
          return None
      return max(strings, key=len)