addition: destroy graph

src/graph.c

@@ -34,3 +34,21 @@ Graph *graph_create(GraphType type) {
 
     return graph;
 }
+
+GraphErr graph_destroy(Graph **graph) {
+    if (graph == NULL || *graph) {
+        return GRAPH_NULL_ARG;
+    }
+
+    size_t vertex_count = arraylist_size((*graph)->vertices);
+
+    for (size_t i = 0; i < vertex_count; i++) {
+        Vertex vertex;
+        arraylist_get((*graph)->vertices, i, &vertex);
+        arraylist_destroy(&vertex.edges);
+    }
+
+    arraylist_destroy(&(*graph)->vertices);
+
+    return GRAPH_OK;
+}