/in/foo.cc: In function 'void SPFA()':
/in/foo.cc:36:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int i = 0; i < G[f].size (); i ++){
                         ~~^~~~~~~~~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:79:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             for (int j = 0; j < G[f].size (); j ++){
                             ~~^~~~~~~~~~~~~~