returns the Jacobian for the position of an SPFA

两种实用性非常强的最短路算法，一个是dijkstra算法，还有一个是比较新的spfa算法，全都自带输出格式，推广性很强。-Two kinds of practical, very strong shortest path algorithm, one is dijkstra algorithm, there is a relatively new spfa algorithm, all carrying the output format to promote strong.