CSC301: Tree code: Recursion going left and right [35/56]

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

Add in only once.

Here is a trace of an execution.

What is the initial value of sz at each node as we go around the tree?