As several people upstairs said, this is a recursive call. For example, x= 123 in the main function, then void is passed in for the first time.

f(int

N) Yes 123. Next, look at the implementation of f( 123). First, judge whether the number of passed-in values is less than 10, and the answer is definitely not. Then enter the contents of else. For the first time, you understand that you took a single digit, that is, the first digit, and output it. Then this F function calls itself and executes again. At this point, so back and forth. The last one is when you don't know 10. Just output it directly. okay