广度优先 Given a graphG=(,E)and a distinguished source vertex s, breadth-first search systematiclly explores the edges of G to discover every vertex that is reachable from s, It computes the distance(smallest number of edges) from s to each reachable vertex. It also produces a"breadth-first tree" with root s that contains all reachable vertices. For any vertex v reachable from s, the simple path on both directed and undirected graphs 两关键动H0W?广度优先 两组关键的动词 How？