1. 将 `vis[1]=1` 去掉，不然触发`if(vis[x])continue;`。
by zjpwdyf @ 2024-03-02 16:39:40
2. `if(d[y]>d[x]+1){` 中仍需要加一句 `ans[y]=ans[x]`
by zjpwdyf @ 2024-03-02 16:40:24
3. 2e6 条双向边，显然 `a` 数组要开到 4e6，不然会 RE
by zjpwdyf @ 2024-03-02 16:41:48
经过这些修改后可以 [AC](https://www.luogu.com.cn/record/149020768) (我小号)。
by zjpwdyf @ 2024-03-02 16:42:46
@[I_AK_IOI_1114](/user/772368)
by zjpwdyf @ 2024-03-02 16:43:07
@[zjpwdyf](/user/807826) dalao好强 关了
by I_AK_IOI_1114 @ 2024-03-04 17:49:46