单源最短路径Bellman-ford。该算法可以用来解决一般（边的权值为负）的单源最短路径问题，而dijkstra只能解决权值非负的情况。此算法使用松弛技术，对每一个顶点，逐步减少源到该顶点的路径的估计值，直到达到最短的路径。-Single source shortest path Bellman-ford. The algorithm can be used to solve the general (the right side is negative) single-source shortest path problem, and dijkstra can only solve the case of non-negative weights. This algorithm uses relaxation techniques, for each vertex, to gradually reduce the source to the vertex of the estimated value until the shortest path.