定向策略：若 $s=a_i$ 或 $s=b_i$，则从编号较大的节点指向编号较小的节点（记这些边为黑色边），否则反之。
只需找到满足“与该节点相连的所有边都是黑色边”的节点，$s$ 即为其编号，若不存在这样的节点则可得 $s=n+1$。
QOJ.ac
QOJ
The 3rd Universal Cup Finals is coming! Join our Warm-up Game and Prediction Game and win the prizes! Learn more...
Discussion #1597 for Problem #17776. 流光解密
Type: Editorial
Status: Open
Posted by: Anonymous
Posted at: 2026-04-22 17:11:03
Last updated: 2026-04-22 17:11:11
Official Editorial
Comments
No comments yet.