foo.cc: In function 'VI sub(VI&, VI&)':
foo.cc:20:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         if (i < b.size()) t -= b[i];
             ~~^~~~~~~~~~
foo.cc: In function 'VI add(VI&, VI&)':
foo.cc:32:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         if (i < a.size()) t += a[i];
             ~~^~~~~~~~~~
foo.cc:33:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         if (i < b.size()) t += b[i];
             ~~^~~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务