Method contains

publication/code/chapter09/word_ladder.rs:102–108

(&self, key: &T)

    }

    fn contains(&self, key: &T) -> bool {
        for (nbr, _vertex) in self.vertices.iter() {
            if nbr == key { return true; }
        }

        false
    }

    // 添加点
    fn add_vertex(&mut self, key: &T) -> Option<Vertex<T>> {