foo.cc: In function 'int muti(std::vector<int>&, std::vector<int>&)':
foo.cc:9:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for(int i = 0; i < x.size(); i ++)
                   ~~^~~~~~~~~~
foo.cc:10:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for(int j = 0; j < y.size(); j ++)
                        ~~^~~~~~~~~~

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