def reverse(str):
    res = ''
    def _reverse():
        nonlocal res
        nonlocal str
        if len(str) > 0:
            res += str[-1]
            str = str[:-1]
            _reverse()
    _reverse()
    return res
print(reverse("abcd"))

 def reverse(s):
    if len(s) == 1:
        return s
    res = s[-1] + reverse(s[:-1])
    return res
print(reverse("abcd"))