Method rotate_left

code/chapter07/avl.rs:183–190

左右旋

(&mut self)

    // 左右旋
    fn rotate_left(&mut self) {
        let mut v = replace(self, Null);
        let mut right = replace(v.right_subtree(), Null);
        let right_left = replace(right.left_subtree(), Null);
        *v.right_subtree() = right_left;
        *right.left_subtree() = v;
        *self = right;
    }

    fn rotate_right(&mut self) {
        let mut v = replace(self, Null);