Method levelorder

publication/code/chapter08/avl.rs:390–406

(&self)

    }

    fn levelorder(&self) {
        let size = self.size();
        let mut q = Queue::new(size);

        let _r = q.enqueue(self);
        while !q.is_empty() {
            let front = q.dequeue().unwrap();
            match front {
                Null => (),
                Tree(node) => {
                    println!("key: {:?}", node.key);
                    let _r = q.enqueue(&node.left);
                    let _r = q.enqueue(&node.right);
                },
            }
        }
    }
}

// 前中后层序遍历: 外部实现