从图中的某个顶点出发到达另外一个顶点的所经过的边的权重和最小的一条路径，称为最短路径此处运用了SPFA的算法，可以解决单源最短路径(From the vertex of a graph, the shortest path is the weight and the smallest path to the edge of another vertex.Here we use SPFA algorithm to solve single