not faster, it's slower

boj/11725_fast.ml

@@ -0,0 +1,77 @@
+(* this is not faster; this is actually slower!!!! *)
+module HashSet = struct
+  type t = (int, int option) Hashtbl.t
+
+  let create () : t = Hashtbl.create 12
+  let add (s : t) x = Hashtbl.add s x None
+  let iter (s : t) ~f = Hashtbl.iter (fun node _ -> f node) s
+
+  let singleton x =
+    let t = create () in
+    add t x;
+    t
+  ;;
+end
+
+module Graph = struct
+  type t = (int, HashSet.t) Hashtbl.t
+
+  let create () : t = Hashtbl.create 16
+
+  let add_edge (g : t) src snk =
+    match Hashtbl.find_opt g src with
+    | Some edge -> HashSet.add edge snk
+    | None -> Hashtbl.add g src (HashSet.singleton snk)
+  ;;
+
+  let dfs_iter g src ~(parent : int option) ~f =
+    let visited = Hashtbl.create 16 in
+    let has_visited x = Hashtbl.mem visited x in
+    let visit ~parent x =
+      f parent x;
+      Hashtbl.add visited x None
+    in
+    let rec helper ~parent src =
+      if has_visited src
+      then ()
+      else (
+        visit ~parent src;
+        match Hashtbl.find_opt g src with
+        | None -> ()
+        | Some edge ->
+          HashSet.iter edge ~f:(fun node -> helper ~parent:(Some src) node))
+    in
+    helper ~parent src
+  ;;
+end
+
+let load () =
+  let n = Scanf.scanf "%d " Fun.id in
+  let g = Graph.create () in
+  for _ = 1 to n - 1 do
+    let src, snk = Scanf.scanf "%d %d " (fun x y -> x, y) in
+    Graph.add_edge g src snk;
+    Graph.add_edge g snk src
+  done;
+  g, n
+;;
+
+let () =
+  let graph, n = load () in
+  let parent_map = Hashtbl.create 16 in
+  Graph.dfs_iter
+    graph
+    ~parent:None
+    ~f:(fun parent src ->
+      match parent with
+      | None -> ()
+      | Some p -> Hashtbl.add parent_map src p)
+    1;
+  for s = 2 to n do
+    Printf.printf
+      "%d\n"
+      (match Hashtbl.find_opt parent_map s with
+       | None -> -1
+       | Some x -> x)
+  done
+;;