判定有向图是否存在回路
输入约定
第一行为图中顶点的个数n； 第二行为途中弧度条数e；
第二行为顶点信息;接着e行为e条弧依附的两个顶点。(Determine whether there is a loop in a directed graph
Input Convention
The number of the vertices in the first behavior graph is n; second the number of radians in the way of behavior is e;
Second behavior vertex information; then the two vertices of the e behavior e arc attachment)