And the thing I wanted point out is, we've got this global variable number of calls. Which is there not because Fibonacci needs it but just for pedagogical reasons, so that we can keep track of how much work this thing is doing.

要指出的是我们已经,调用了几次这个全局变量,这不是因为斐波那契数列,需要调用它而只是,因为教学上的需要，以便我们可以知道,这个函数做了多少事情。

麻省理工公开课 - 计算机科学及编程导论课程节选