public Graph toMinSpanTree() {
		PriorityQueue<Edge> queue = new PriorityQueue<Edge>();
		UnionFindSet makeSet = new UnionFindSet();
		DiGraph graph= new DiGraph();	// MST

		makeSet.add(getNodes()); //generate 1-elements-sets

		//adding all edges into the queue
		for (Node node : nodes.values()) {
			List<Edge> incidentEdges = node.getIncidentEdges();
			queue.addAll(incidentEdges);
		}
		for(Edge e : queue) {
			if(makeSet.getRepresentative(e)!=makeSet.getRepresentative(e)) {
				//edge connects different sets
				makeSet.union(e, e);;
				graph.addEdge(0, 0, 0);
			}

		}// end for

		return new DiGraph();
	}