input: def sort_by_length(strings):
      """
      Sort a list of strings by their length in ascending order using a simple bubble sort algorithm.
      
      >>> sort_by_length(["cat", "dog", "apple", "banana"])
      ['cat', 'dog', 'apple', 'banana']
      >>> sort_by_length(["python", "java", "c", "swift"])
      ['c', 'java', 'swift', 'python']
      """
output:     n = len(strings)
      for i in range(n):
          for j in range(0, n-i-1):
              if len(strings[j]) > len(strings[j+1]):
                  strings[j], strings[j+1] = strings[j+1], strings[j]
      return strings