Method get_vertex

publication/code/chapter09/graph_adjlist.rs:95–101

(&self, key: &T)

    }

    fn get_vertex(&self, key: &T) -> Option<&Vertex<T>> {
        if let Some(vertex) = self.vertices.get(key) {
            Some(&vertex)
        } else {
            None
        }
    }

    // 获取所有节点的 key
    fn vertex_keys(&self) -> Vec<T> {