CSC301: Tree code: Base case first (negating the conditional) [37/56]

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

Base case first (negating the conditional)

This is more idiomatic for recursive functions.