foo.cc: In function 'std::__cxx11::string add(std::__cxx11::string, std::__cxx11::string)':
foo.cc:22:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i = 0 ; i < t1.length() ; i++){
                     ~~^~~~~~~~~~~~~
foo.cc:40:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i = 0 ; i < ans.length() ; i++){
                     ~~^~~~~~~~~~~~~~
foo.cc:21:9: warning: unused variable 'len' [-Wunused-variable]
     int len =t1.length();
         ^~~

自豪的采用jd5进行评测(github.com/masnn/jd5)

{"receive":"2020-02-23T06:28:58.596Z","handle":"2020-02-23T06:28:58.596Z","cache_start":"2020-02-23T06:28:58.597Z","read_cases":"2020-02-23T06:28:58.787Z","judge":"2020-02-23T06:28:58.788Z","done":"2020-02-23T06:28:59.496Z"}