foo.cc: In function 'int main()':
foo.cc:16:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<a.length();i++){
                 ~^~~~~~~~~~~
foo.cc:32:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<a.length();i++){
                 ~^~~~~~~~~~~
foo.cc:7:9: warning: variable 'flag' set but not used [-Wunused-but-set-variable]
     int flag = 0;
         ^~~~

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