Method left_subtree

code/chapter07/avl.rs:168–173

获取左右子树

(&mut self)

    // 获取左右子树
    fn left_subtree(&mut self) -> &mut Self {
        match *self {
            Null => panic!("Empty tree"),
            Tree(ref mut node) => &mut node.left,
        }
    }

    fn right_subtree(&mut self) -> &mut Self {
        match *self {