将doTowers的返回类型从void更改为int，并将返回值设置为:

Change the return type of doTowers from void to int, and set the return value to:

1. 如果topN == 1，则返回1；
2. 否则返回两个doTowers()加1的总和.

1. if topN == 1, return 1;
2. else return the sum of two doTowers() plus 1.

逻辑类似于问题的算法.玩弄找出来吧！

The logic is similar to the algorithm of the problem. Have fun figuring it out!

您还可以使用静态全局变量，但这可以说是不好的编程风格.

You could also use a static global variable, but that's arguably bad programming style.