1 state may be typedef type, which only represents the success or failure of insertion, and it can be completely replaced by bool type, which has no influence on the function;

2 Linked list & ampl represents a reference to L. If the reference is used as a parameter, the parameter is not "value passing". If the value of the formal parameter in the function changes, the value of the referenced variable will also change. C language supports citation;

3 void main () may compile, but it is not standardized and unsafe, and int main () is fine.