foo.cc: In function 'std::__cxx11::string qth(std::__cxx11::string)':
foo.cc:15:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<n.size();i++)
                 ~^~~~~~~~~
foo.cc: In function 'void pd(std::__cxx11::string)':
foo.cc:78:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<t.size()-1;i++)
                 ~^~~~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:95:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<n.size();i++)//后缀表达式计算
                 ~^~~~~~~~~

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