Neither Eular circuit nor Eular path exists in this graph, since there are more than two vertices with odd degree.
这个图既没有欧拉回路，也没有欧拉路径。因为有超过两个顶点的度数为奇数。
2
If the two endpoints of a route are the same node, the route is a circuit; if the two endpoints of a path are the same node, the path is a unidirectional circuit.
当路径的两个端点重合时就是回路，通路的两个端点重合时就是单向回路。
3
Traditionally, such a circuit was provisioned through the use of two copper pairs, one for transmission ( forward path , or upstream path ) and one for reception ( reverse path , or downstream path );