input: Write a function to reverse a string using recursion.
  assert reverseString("thequickbrownfox") == "xofnworbkciuqeht"
  assert reverseString("hello") == "olleh"
  assert reverseString("python") == "nohtyp"
output: def reverseString(s):
      if len(s) == 0:
          return s
      else:
          return s[-1] + reverseString(s[:-1])