input: Write a python function to reverse a string recursively.
  assert reverse_string("hello") == "olleh"
  assert reverse_string("world") == "dlrow"
  assert reverse_string("python") == "nohtyp"
output: def reverse_string(s):
      if len(s) == 0:
          return s
      return s[-1] + reverse_string(s[:-1])