foo.cc: In function 'int main()':
foo.cc:153:30: warning: format '%d' expects argument of type 'int', but argument 2 has type 'std::vector<int>::size_type {aka long unsigned int}' [-Wformat=]
     printf("%d",answer.size());
                 ~~~~~~~~~~~~~^

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