foo.cc: In function 'std::vector<int> mul(const std::vector<int>&, int)':
foo.cc:21:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < a.size() || ad; i ++)
                  ~~^~~~~~~~~~
foo.cc:23:9: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if (i < a.size()) ad += a[i] * b;
       ~~^~~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:62:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < strlen(n); i ++ )
                  ~~^~~~~~~~~~~

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