Method rotate_right

code/chapter07/avl.rs:192–199

(&mut self)

    }

    fn rotate_right(&mut self) {
        let mut v = replace(self, Null);
        let mut left = replace(v.left_subtree(), Null);
        let left_right = replace(left.right_subtree(), Null);
        *v.left_subtree() = left_right;
        *left.right_subtree() = v;
        *self = left;
    }

    // 树节点数是左右子树节点数加根节点数，递归计算
    fn len(&self) -> usize {