Method cmp

code/chapter08/dijkstra.rs:25–27

(&self, other: &Self)

// 为 Visited 添加全序比较功能
impl<V> Ord for Visited<V> {
    fn cmp(&self, other: &Self) -> Ordering {
        other.distance.cmp(&self.distance)
    }
}

impl<V> PartialOrd for Visited<V> {