Method postorder

publication/code/chapter08/bst.rs:284–294

(&self)

    }

    fn postorder(&self) {
        match &self.left {
            Some(node) => node.postorder(),
            None => (),
        }
        match &self.right {
            Some(node) => node.postorder(),
            None => (),
        }
        println!("key: {:?}, val: {:?}", self.key.unwrap(), self.val.unwrap());
    }

    fn levelorder(&self) {
        let size = self.size();