图论习题2

继续上次学习i:

1.15 ： 不一定正确。可以是多个圈的并，如果连通，则正确！
图论圈的标准定义：
a cycle in a graph is a non-empty trail in which the only repeated vertices are the first and last vertices.
可进一步思考：A graph whose vertices all have degree $2$2 must contain a cycle