CSC403: More Size: Student code [3/10]

    public int size () {
        return size (root);
    }
    private static int size (Node x) {
        if (x == null) return 0;
        int szl = size (x.left.left) + size (x.left.right) + 1;
        int szr = size (x.right.left) + size (x.right.right) + 1;
        return szl + szr + 1;
    }

Is this correct?

Can it be improved?