foo.cc: In function 'int main()':
foo.cc:13:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<x.size();i++)
              ~^~~~~~~~~
foo.cc:15:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<x.size();i++)
              ~^~~~~~~~~
foo.cc:17:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<x.size();i++)
              ~^~~~~~~~~
foo.cc:18:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int j=i+1;j<x.size();j++)
                 ~^~~~~~~~~
foo.cc:21:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<x.size();i++)
              ~^~~~~~~~~
foo.cc:22:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int j=i+1;j<x.size();j++)
                 ~^~~~~~~~~

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