input: Write a function to sort a list of strings by their length in ascending order. If the list is empty, return the empty list.
  
  def sort_by_length(strings):
      for i in range(len(strings) - 1):
          for j in range(i + 1, len(strings)):
              if len(strings[i]) > len(strings[j]):
                  strings[i], strings[j] = strings[j], strings[i]
      return strings
  
  assert sort_by_length(["cat", "dog", "apple", "banana"]) == ["cat", "dog", "apple", "banana"]
  assert sort_by_length([]) == []
  assert sort_by_length(["a", "abc", "ab"]) == ["a", "ab", "abc"]
  assert sort_by_length(["z", "y", "x", "w"]) == ["z", "y", "x", "w"]
  assert sort_by_length(["test", "car", "plane"]) == ["car", "test", "plane"]
output: def sort_by_length(strings):
      return sorted(strings, key=len)