递归计算Ackermenn函数

https://pintia.cn/problem-sets/12/problems/355

int Ack(int m, int n)
{
    int ret;

    if (m == 0)
    {
        ret = n + 1;
    }
    else if (n == 0 && m > 0)
    {
        ret = Ack(m - 1, 1);
    }
    else if (m > 0 && n > 0)
    {
        ret = Ack(m - 1, Ack(m, n - 1));
    }

    return ret;
}