if h <= 5min and idea is None:
    return look up answer and it`s ok.
if h < 1h and solve:
    return good and happy!
if h < 5h and some ideas and not solve:
    return don`t want talk
if h > 5h and much ideas and not solve:
    return let me die

递归两步骤
1. 解决基本问题. (0, 1, null...)
2. 解决一般情况. 一般情况 => 更小子集 (用有限步骤)

解决这两个问题就搞定了, 大大的简化了问题, 而且很容易写