/in/foo.cc: In function 'void spfa1()':
/in/foo.cc:24:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=0;i<tail[x].size();i++)
               ~^~~~~~~~~~~~~~~
/in/foo.cc:35:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int j=0;j<head[i].size();j++)
              ~^~~~~~~~~~~~~~~
/in/foo.cc: In function 'void spfa2()':
/in/foo.cc:51:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=0;i<head[x].size();i++)
               ~^~~~~~~~~~~~~~~