foo.cc: In function 'void spfa()':
foo.cc:40:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int i = 0; i < G[u].size(); i++) {
                   ~~^~~~~~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:54:8: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf ("%d %d %d", &n, &m, &s);
  ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
foo.cc:56:9: 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。