CSC301: Printing: Print prefix [21/30]

    public void printPre () {
        printPre (root, 0);
    }
    private static void printPre (Node x, int d) {
        if (x == null) return;
        indent (d);
        StdOut.println (x.key);
        printPre (x.left, d+1);
        printPre (x.right, d+1);
    }
    private static void indent (int d) {
        for (int i=0; i<d; i++)
            StdOut.print ("  ");
    }

This is Pretty Printing where we use indentation to show depth.