CSC301: Tree code: Back to sanity! [42/56]

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

Threaded parameter: correct version, doing the addition postorder.

What are the initial values of sz as you go around the tree?