input: def sort_by_length(strings: List[str]) -> List[str]:
      """
      Sort a list of strings by their length in ascending order.
      
      >>> sort_by_length(["cat", "dog", "apple", "banana"])
      ["cat", "dog", "apple", "banana"]
      >>> sort_by_length(["elephant", "rat", "lion", "mouse"])
      ["rat", "lion", "mouse", "elephant"]
      """
output:     return sorted(strings, key=len)