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 compone