foo.cc: In function ‘void dfs(int)’:
foo.cc:25:22: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::vector<int>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   25 |         for(int i=0;i<a[u].size();i++)
      |                     ~^~~~~~~~~~~~