// a standard implementation of Dijkstra's algorithm using a priority queue // with a minor modification for the problem static int GetTreeWeight(Graph graph, int hospital, int verticeCount, IEnumerable<int> hospitals) { var distances = new PriorityQueue<int, int>((e1, e2) => -e1.CompareTo(e2)); foreach (var adj in graph[hospital]) { distances.Enqueue(adj.Item2, adj.Item1); } // return value int ret = 0; int edgeCount = 0; while (edgeCount < verticeCount - hospitals.Count()) { edgeCount += 1; // edge nearest to 'hospital' var min = distances.DequeueWithPriority(); var weight = min.Item1; // the new vertex in the tree var v1 = min.Item2; // modification of algorithm: // sum the distance to the root of all nodes that aren't // hospitals if (!hospitals.Contains(v1)) ret += weight; // update the priorities of all edges incident on the vertex // we've just added foreach (var adj in graph[v1]) { var v2 = adj.Item1; if (v2 == hospital) continue; var priority = distances.PriorityOrDefault(v2, int.MaxValue); if (priority > weight + adj.Item2) distances.Rekey(v2, weight + adj.Item2); } } return ret; }