Method add_vertex

publication/code/chapter09/graph_adjlist.rs:89–93

(&mut self, key: &T)

    }

    fn add_vertex(&mut self, key: &T) -> Option<Vertex<T>> {
        let vertex = Vertex::new(key.clone());
        self.vertnums += 1;
        self.vertices.insert(key.clone(), vertex)
    }

    fn get_vertex(&self, key: &T) -> Option<&Vertex<T>> {
        if let Some(vertex) = self.vertices.get(key) {