Notice one last piece. If I'm going to do that, run through all possible answers, I still want to know, gee, what if there aren't any answers? How do I return that case?

注意最后一段，如果我要这样做，也就是运行所有可能的答案，我还是想知道要是没有结果呢？,在这个例子中应该返回什么呢？

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