foo.cc: In function 'std::vector<int> mul(std::vector<int>, std::vector<int>)':
foo.cc:26:16: 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:51:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (i = 0; i < s.length(); i++)
              ~~^~~~~~~~~~~~

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