foo.cc: In function 'void dfs(int)':
foo.cc:7:21: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    7 |   for (int i = 0; i < G[f].size(); ++i) {
      |                   ~~^~~~~~~~~~~~~

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