// TREE code

    // prefix order traversal
    public void printPre () {
        if (root != null) printPre (root);
        StdOut.println ();
    }
    private static void printPre (Node n) {
        StdOut.print (n.val + " ");

        for (Node child : n.children) {

            printPre (child, marked);
        }
    }

     // level order traversal
     public void printLevel () {
         Queue<Node> queue = new Queue<>();

         if (root != null) queue.enqueue(root);
         while (!queue.isEmpty()) {
             Node n = queue.dequeue();
             StdOut.print (n.val + " ");
             for (Node child : n.children) {

                 queue.enqueue(child);
             }
         }
         StdOut.println ();
     }
    }

    // GRAPH code

    // preorder traversal
    public void printPre () {
        if (root != null) printPre (root, new HashSet<Node> ());
        StdOut.println ();
    }
    private static void printPre (Node n, HashSet<Node> marked) {
        StdOut.print (n.val + " ");
        marked.add (n);
        for (Node child : n.children) {
            if (!marked.contains (child))
                printPre (child, marked);
        }
    }

    // level order traversal
    public void printLevel () {
        Queue<Node> queue = new Queue<> ();
        HashSet<Node> marked = new HashSet<> ();
        if (root != null) { queue.enqueue(root); marked.add (root); }
        while (!queue.isEmpty()) {
            Node n = queue.dequeue();
            StdOut.print (n.val + " ");
            for (Node child : n.children) {
                if (!marked.contains (child))
                    { queue.enqueue(child); marked.add (n); }
            }
        }
        StdOut.println ();
    }