foo.cc: In function 'std::string dfs(std::vector<std::vector<int> >, int)':
foo.cc:13:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   13 |     for (int i = 0; i < tree[root].size(); ++i)
      |                     ~~^~~~~~~~~~~~~~~~~~~

正在同步测试数据，请稍后
[Hydro](https://hydro.ac)提供评测服务