For this question, I would suggest that you make a diagram to help you analyze it.

对这道题，我建议首先画一个图来帮助你分析。

youdao

youdao

I personally like to include return messages whenever a value will be returned, because I find the extra details make a sequence diagram easier to read.

我个人喜欢，无论什么时候返回一个值，都包括一个返回消息，因为我发现额外的细节使一个序列图变得更容易阅读。

youdao

Mr Putin even drew a diagram on a napkin to make his point.

ECONOMIST: Russia and NATO