Method node

code/chapter07/avl.rs:160–165

获取节点

(&mut self)

    // 获取节点
    fn node(&mut self) -> &mut AvlNode<T> {
        match *self {
            Null => panic!("Empty tree"),
            Tree(ref mut n) => n,
        }
    }

    // 获取左右子树
    fn left_subtree(&mut self) -> &mut Self {