How to rewrite Ackermann function in non-recursive style?

Question

I have function

public static int func(int M,int N){
    if(M == 0 || N == 0) return M+N+1;
    return func(M-1, func(M, N-1));
}

How to rewrite it in non-recursive style ? Maybe, is it implementation some algorithm?

Answer 1

Not quite O(1) but definitely non-recursive.

public static int itFunc(int m, int n){
    Stack<Integer> s = new Stack<Integer>;
    s.add(m);
    while(!s.isEmpty()){
        m=s.pop();
        if(m==0||n==0)
            n+=m+1;
        else{
            s.add(--m);
            s.add(++m);
            n--;
        }
    }
    return n;
}

Answer 2

All the answers posted previously don't properly implement Ackermann.

def acker_mstack(m, n)
  stack = [m]
  until stack.empty?
    m = stack.pop

    if m.zero?
      n += 1
    elsif n.zero?
      stack << m - 1
      n = 1
    else
      stack << m - 1 << m
      n -= 1
    end
  end
  n
end