foo.cc: In function 'long long int visit(int, int)':
foo.cc:17:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   17 |     for (int i = 0; i < vec[pre].size(); i++)
      |                     ~~^~~~~~~~~~~~~~~~~

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