foo.cc: In function 'void mergesort(int)':
foo.cc:27:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for(int j=0;j<v[i].size();j++)
                ~^~~~~~~~~~~~
foo.cc:37:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for(int j=1;j<v1[i].size();j++)
                ~^~~~~~~~~~~~~

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