foo.cc: In function 'void dfs(int)':
foo.cc:76:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<a1[x].size();i++) c1[a1[x][i]]++;
                 ~^~~~~~~~~~~~~
foo.cc:77:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<b1[x].size();i++) c1[b1[x][i]]--;
                 ~^~~~~~~~~~~~~
foo.cc:78:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<a2[x].size();i++) c2[a2[x][i]+n]++;
                 ~^~~~~~~~~~~~~
foo.cc:79:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<b2[x].size();i++) c2[b2[x][i]+n]--;
                 ~^~~~~~~~~~~~~

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