examples: add more graphs examples, fix typo (#17113)

examples/graphs/bfs2.v

@@ -60,9 +60,8 @@ fn departure(mut queue []string) string {
 // Creating aa map to initialize with of visited nodes .... all with false in the init
 // so these nodes are NOT VISITED YET
 fn visited_init(a_graph map[string][]string) map[string]bool {
-	mut array_of_keys := a_graph.keys() // get all keys of this map
 	mut temp := map[string]bool{} // attention in these initializations with maps
-	for i in array_of_keys {
+	for i, _ in a_graph {
 		temp[i] = false
 	}
 	return temp