遍历一遍，判断图分为几部分（假定为P部分，即图有 P 个连通分量）,对于每一个连通分量，如果无环则只能是树，即：边数＝结点数－1,只要有一个满足边数>结点数－1,原图就有环.

-Traverse again, the determination map is divided into several parts (assumed to section P, with P in Figure connected component), for each connected component, if it can be pierced tree, namely: the number of edges = Junction points-1, as long as There are a number of edges meet>　nodes-1, artwork alone ring.