CSC301: Tree code: Threaded parameter with non-nullable pointer 1 [55/56]

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

Describe the difference from the previous version.

Is it correct?

What's the invariant as we go through the tree?