foo.cc: In function 'int main()':
foo.cc:21:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(i=0;i<a.length();i++){
             ~^~~~~~~~~~~
foo.cc:22:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             for(j=0;j<b.length();j++){
                     ~^~~~~~~~~~~
foo.cc:26:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(i=0;i<a.length()+b.length();i++){
             ~^~~~~~~~~~~~~~~~~~~~~~

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