说明：  单源最短路径问题：给定带权有向图G=(V,E)。给定V中的一个顶点v，称为源。要计算从源到所有其它各顶点的最短路径长度。
(Single-source shortest path problem: a given weighted directed graph G = (V, E). Given a vertex V in v, known as the source. To calculate from the source to all other vertices of the shortest path length.)