foo.cc: In function 'void solve(std::vector<int>&&)':
foo.cc:27:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   27 |     for (int i = 0; i < idx.size(); i++) cnt[col[idx[i]]].push_back(i);
      |                     ~~^~~~~~~~~~~~

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