input: Write a function to sort a list of strings by their lengths in ascending order. The function should take a list of strings as input and return the sorted 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(["apple", "orange", "cherry", "banana"])==["cherry", "apple", "orange", "banana"]
  assert sort_by_length(["a", "aa", "aaa", "aaaa"])==["a", "aa", "aaa", "aaaa"]
  assert sort_by_length(["python", "java", "c", "ruby"])==["c", "ruby", "java", "python"]
  assert sort_by_length([])==[]
  
output: def sort_by_length(strings):
      return sorted(strings, key=len)