CSC301: Printing: Print infix [23/30]

    public void printIn () {
        printIn (root);
        StdOut.println ();
    }
    private static void printIn (Node x) {
        if (x == null) return;
        printIn (x.left);
        StdOut.print (x.key + " ");
        printIn (x.right);
    }

Yeah!