foo.cc: In function 'int scan(std::__cxx11::string, int, int, int&, int&)':
foo.cc:11:1: warning: no return statement in function returning non-void [-Wreturn-type]
 }
 ^
foo.cc: In function 'int main()':
foo.cc:27:9: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  while(t<=a.length()){
        ~^~~~~~~~~~~~
foo.cc:41:9: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  while(t<=a.length()){
        ~^~~~~~~~~~~~

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。