Iterative Implementation of Ackermann Function

ackermann.py

import unittest

UNKNOWN = None

def ackermann(m, n):
    last_answer = None
    
    stack = []
    stack.append((m, n))
    while len(stack) > 0:
        cur_m, cur_n = stack.pop()

        if cur_n == UNKNOWN:
            stack.append((cur_m, last_answer))
        elif cur_m == 0:
            last_answer = cur_n+1
        elif cur_n == 0:
            stack.append((cur_m-1, 1))
        else:
            stack.append((cur_m-1, UNKNOWN))
            stack.append((cur_m, cur_n-1))
    return last_answer

class AckermannTest(unittest.TestCase):
    def test_ackermann(self):
        for n in range(10):
            self.assertEqual(ackermann(0, n), n+1)

        for n in range(10):
            self.assertEqual(ackermann(1, n), 2+(n+3)-3)

        for n in range(10):
            self.assertEqual(ackermann(2, n), 2*(n+3)-3)

        for n in range(9):
            self.assertEqual(ackermann(3, n), 2**(n+3)-3)

        self.assertEqual(ackermann(4, 0), 13)

if __name__ == "__main__":
    unittest.main()