input: Write a python function to reverse a string using a list.
  assert reverse_string("Hello world") == "dlrow olleH"
  assert reverse_string("Python") == "nohtyP"
  assert reverse_string("OpenAI") == "IAnepO"
output: def reverse_string(s):
      char_list = list(s)
      reversed_list = char_list[::-1]
      return ''.join(reversed_list)