输入为一个有向图。边数为n，每条边的起点、终点均需输入。判断该有向图中是否存在拓扑结构，如果存在则输出，如果不存在则输出该有向图中的圈。-Enter as a directed graph. The number of edges is n, the starting point of each edge, end required input. The judge has the topology of the existence of the figure, if there is outp