RosettaCodeData/Task/Ackermann-function/Python/ackermann-function-2.py

from functools import lru_cache

@lru_cache(None)
def ack2(M, N):
    if M == 0:
        return N + 1
    elif N == 0:
        return ack2(M - 1, 1)
    else:
        return ack2(M - 1, ack2(M, N - 1))