/**
      * Prints out topological ordering of the graph given
      * @param graph graph
      */
     public static void orderTopologically(GraphIface graph) {
         //states of particular nodes
         List<Integer> roots = new ArrayList<Integer>();
         for (int i = 0; i < graph.getVertexCount(); i++) {
             //if it does not have any descendants in original graph
             //than it is a root in the inverse graph
             if(graph.getEdges(i).size() == 0) roots.add(i);
         }
         GraphIface inverted = graph.inverse(); //create inversly oriented graph
         int[] state = new int[inverted.getVertexCount()];
 
         for (int i = 0; i < state.length; i++) state[i] = FRESH;
         for (Integer i : roots) {
             doOrdering(inverted, i, state);
         }
     }
     /**
      * Perform the topological ordering (sorting)
      * @param graph graph
      * @param vertexNr number of the current node
      * @param state array of states
      */
     private static void doOrdering(GraphIface graph, int vertexNr, int[] state) {
         state[vertexNr] = OPENED;
         for (Integer i : graph.getEdges(vertexNr)) {
             if (state[i] == FRESH) doOrdering(graph, i, state);
             else if (state[i] == OPENED) throw new IllegalArgumentException("Graph contains cycles");
         }
         System.out.print(" -> " + vertexNr);
         state[vertexNr] = CLOSED;
     }