CSC403: Printing: Print prefix with a loop [9/10]

    public void printPre () {
        Stack<Node> s = new Stack<> ();
        s.push (root);
        while (!s.isEmpty ()) {
            Node x = s.pop ();
            if (x == null) continue;
            StdOut.print (x.key + " ");
            s.push (x.right);
            s.push (x.left);
        }
        StdOut.println ();
    }

Print level-order in a loop: not bad