foo.cc: In function 'std::vector<int> mul(std::vector<int>, std::vector<int>)':
foo.cc:24:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (i = 0; i < c.size(); i++)
              ~~^~~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:44:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (i = 0; i < s.length(); i++) {
              ~~^~~~~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务