// level order traversal of a tree

    public void printLevel () {
        Queue<Node> q = new Queue<> ();

        q.enqueue (root);
        while (!q.isEmpty ()) {
            Node x = q.dequeue ();
            if (x == null) continue;
            StdOut.print (x.key + " ");
            q.enqueue (x.left);
            q.enqueue (x.right);
        }
        StdOut.println ();
    }

    // breadth first traversal of a graph
    // mark when enqueuing
    public void printLevel () {
        Queue<Node> q = new Queue<> ();
        HashSet<Node> marked = new HashSet<> ();
        q.enqueue (root); marked.add (root);
        while (!q.isEmpty ()) {
            Node x = q.dequeue ();
            if (x == null || marked.contains (x)) continue;
            StdOut.print (x.key + " ");
            q.enqueue (x.left);  marked.add (x.left)
            q.enqueue (x.right); marked.add (x.right)
        }
        StdOut.println ();
    }