```cpp
n = 4, m = 4
1 2
2 3
3 4
4 2
```
by yukimianyan @ 2024-04-13 12:04:42
@[TernaryTree](/user/362750) @[Drind](/user/305854) @[gesong](/user/658198) @wwqwq @[codingwen](/user/736237) @[what_can_I_do](/user/658973)
by YuRuochen @ 2024-04-13 12:05:15
反例：一个节点指向一个环
by wangbinfeng @ 2024-04-13 12:05:28
@[wwqwq](/user/669919)
by YuRuochen @ 2024-04-13 12:05:37
这显然是不对的吧？？
by Fractured_Angel @ 2024-04-13 12:05:54
懒得画图了，考虑甲基苯的键线式
by wangbinfeng @ 2024-04-13 12:07:00
@[yukimianyan](/user/509229) @[wangbinfeng](/user/387009) @[the_long_way_to_dp](/user/837888)
%%%，thx！
by YuRuochen @ 2024-04-13 12:07:22
@[YuRuochen](/user/658786) 你的结论应该是针对且仅针对有向无环图（DAG）得吧？
by wangbinfeng @ 2024-04-13 12:15:23
~~才发现我和lz得咕值一样耶~~
by wangbinfeng @ 2024-04-13 12:16:27
@[wangbinfeng](/user/387009) 嗯。事实上“不存在出度为 $0$ 的点” 和 “不存在入度为 $0$ 的点”貌似都不能在 DAG 里出现。
by YuRuochen @ 2024-04-13 12:21:45