foo.cc: In function 'int main()':
foo.cc:61:34: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
                 for(int i = 0; i < to[b][c].size(); i++) {
                                ~~^~~~~~~~~~~~~~~~~
foo.cc:66:30: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             for(int i = 0; i < from[a][c].size(); i++) {
                            ~~^~~~~~~~~~~~~~~~~~~
foo.cc:40:54: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
         int a = read(), b = read(); char ch[2]; scanf("%s", ch);
                                                 ~~~~~^~~~~~~~~~

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

{"receive":"2020-07-16T07:14:16.006Z","handle":"2020-07-16T07:14:16.006Z","cache_start":"2020-07-16T07:14:16.010Z","read_cases":"2020-07-16T07:14:16.121Z","judge":"2020-07-16T07:14:16.122Z","done":"2020-07-16T07:14:17.414Z"}