/**
     * Generates a HashMap that represents the MTR network with the stations and
     * all their immediate connections.
     *
     * @param lineMap
     *            the lines and their respective stations
     * @return a map of the stations and their respective Node object
     */
    public HashMap<String, Node<String, String>> generateNodesMap(Map<String, String[]> lineMap) {
        // initialise map that stores all the station nodes, with the respective
        // station's name as key
        HashMap<String, Node<String, String>> nodesMap = new HashMap<String, Node<String, String>>(64);

        // iterate through every station line
        for (Entry<String, String[]> pair : lineMap.entrySet()) {
            // retrieve station line
            String line = pair.getKey();
            // retrieve stations
            String[] stations = pair.getValue();

            // declare and initialise a variable to indicate if the current
            // iteration is the first
            boolean first = true;

            // store the node found in the previous iteration to be used in
            // creating an edge
            Node<String, String> prevNode = null;

            // iterate through the stations
            Iterator<String> itr = Arrays.asList(stations).iterator();
            while (itr.hasNext()) {
                // retrieve the next station
                String station = itr.next();

                // retrieve already existing node of the station, or create a
                // new one
                Node<String, String> node;
                if (nodesMap.containsKey(station)) {
                    node = nodesMap.get(station);
                } else {
                    node = new Node<String, String>(station);
                }

                // check if it is the first iteration
                if (first) {
                    // ignore creating an edge between a (non-existent) previous
                    // node and tell the subsequent iterations that the first
                    // iteration has occurred
                    first = false;
                    // create edge between current node and previous node
                } else {
                    // create a new edge between two nodes
                    Edge<String, String> edge = new Edge<String, String>(prevNode, node, line);
                    // add the generated edge to the two stations
                    prevNode.addEdge(edge);
                    node.addEdge(edge);
                }
                // add the station to the nodes map with the key as the
                // station's name
                nodesMap.put(station, node);

                // store the previously modified station
                prevNode = node;
            }
        }

        return nodesMap;
    }