Method postorder

code/chapter07/bst.rs:74–84

(&self)

    }

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

    fn insert(&mut self, key: T, val: U) {
        // 没数据直接插入