added Vertices() to Graph interface

directed.go

@@ -62,6 +62,10 @@ func (d *directed[K, T]) Vertex(hash K) (T, error) {
 	return vertex, err
 }
 
+func (d *directed[K, T]) Vertices() ([]K, error) {
+	return d.store.ListVertices()
+}
+
 func (d *directed[K, T]) VertexWithProperties(hash K) (T, VertexProperties, error) {
 	vertex, properties, err := d.store.Vertex(hash)
 	if err != nil {

graph.go

@@ -88,6 +88,10 @@ type Graph[K comparable, T any] interface {
 	// doesn't exist.
 	Vertex(hash K) (T, error)
 
+	// Vertices returns a slice of all vertices in the graph. These vertices are of type
+	// Vertice[K] and hence will contain the vertex hashes, not the vertex values.
+	Vertices() ([]K, error)
+
 	// VertexWithProperties returns the vertex with the given hash along with
 	// its properties or ErrVertexNotFound if it doesn't exist.
 	VertexWithProperties(hash K) (T, VertexProperties, error)

undirected.go

@@ -43,6 +43,10 @@ func (u *undirected[K, T]) Vertex(hash K) (T, error) {
 	return vertex, err
 }
 
+func (u *undirected[K, T]) Vertices() ([]K, error) {
+	return u.store.ListVertices()
+}
+
 func (u *undirected[K, T]) VertexWithProperties(hash K) (T, VertexProperties, error) {
 	vertex, prop, err := u.store.Vertex(hash)
 	if err != nil {