foo.cc: In function 'void solve()':
foo.cc:29:27: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   29 |         for (int i = 0; i < res.size(); i++)
      |                         ~~^~~~~~~~~~~~

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