private void solve(IDirectedGraph g)
        {
            result = new List <int>();
            Degrees degrees_info = new Degrees(g);

            int[] degrees = new int[g.V];
            for (int i = 0; i < g.V; ++i)
            {
                degrees[i] = degrees_info.InDegree(i);
            }
            Queue <int> starts = new Queue <int>();

            foreach (Int32 s in degrees_info.Sources())
            {
                starts.Enqueue(s);
            }
            //只要还有顶点
            while (starts.Count > 0)
            {
                Int32 v = starts.Dequeue();
                result.Add(v);
                foreach (var w in g.Adj(v))
                {
                    //删除这个顶点的指出的边,因为这个顶点已经被删除了
                    --degrees[w];
                    if (degrees[w] == 0)
                    {
                        //下一个拓扑排序的顺序
                        starts.Enqueue(w);
                    }
                }
            }
        }