non-recursive implementation of Ackermann function

ackermann.py

def ack_rec(m, n):
  print(m, n)
  if m == 0:
    return n + 1
  elif n == 0:
    return ack_rec(m-1, 1)
  else:
    return ack_rec(m-1, ack_rec(m, n-1))

def ack_norec(m, n):
  stack = [(m, n, 0)]
  res = 0
  while len(stack) > 0:
    (m, n, r) = stack.pop()
    if r == 1:
      n = res

    if m == 0:
      res = n + 1
    elif n == 0:
      stack.append((m-1, 1, 0))
    else:
      stack.append((m-1, 0, 1))
      stack.append((m, n-1, 0))
  return res