input: Write a python function to reverse a string recursively.
  assert reverse_string_recursive("hello") == "olleh"
  assert reverse_string_recursive("Python") == "nohtyP"
  assert reverse_string_recursive("12345") == "54321"
output: def reverse_string_recursive(s):
      if len(s) == 0:
          return s
      else:
          return reverse_string_recursive(s[1:]) + s[0]