foo.cc: In function 'void dfs(int)':
foo.cc:20:15: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   20 |  for(int i=0;i<a[x].size();i++) dfs(a[x][i]);
      |              ~^~~~~~~~~~~~

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