input: Write a python function to find the longest string in a list of strings.
  assert longest_string(["apple", "banana", "cherry", "date"]) == "banana"
  assert longest_string(["cat", "elephant", "dog", "mouse"]) == "elephant"
  assert longest_string(["a", "ab", "abc", "abcd"]) == "abcd"
output: def longest_string(lst):
      return max(lst, key=len)