foo.cc: In function 'void spfa()':
foo.cc:18:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for(int i=0;i<E[u].size();++i){
                     ~^~~~~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:28:10: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d %d",&n,&m);
     ~~~~~^~~~~~~~~~~~~~~
foo.cc:30:14: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
         scanf("%d %d %d",&u,&v,&w);
         ~~~~~^~~~~~~~~~~~~~~~~~~~~

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