问题2 单源最短略问题的解必定是一棵树？ A shortest-paths tree rooted at s is a directed subgraph G'=(V',E), where V'C V and E'C E,such that 1.V'is the set of vertices reachable from s in G, 2.G'forms a rooted tree with root s,and 3.for all vV,the unique simple path froms to v in G'is a shortest path froms to v in G. 由.T决定的 Vπ={v∈V:w.π≠NTL}U{s}· 最短路径树： E元={v.π，v)∈E:v∈Vπ-{s}由.π决定的 最短路径树：