Problem:
Score : 300 points

Problem Statement

You are given a string S consisting of lowercase English letters.
Another string T is initially empty.
Determine whether it is possible to obtain S = T by performing the following operation an arbitrary number of times:

Append one of the following at the end of T: dream, dreamer, erase and eraser.

Constraints

1≦|S|≦10^5

S consists of lowercase English letters.

Input

The input is given from Standard Input in the following format:

S

Output

If it is possible to obtain S = T, print YES. Otherwise, print NO.

Sample Input 1

erasedream

Sample Output 1

YES

Append erase and dream at the end of T in this order, to obtain S = T.

Sample Input 2

dreameraser

Sample Output 2

YES

Append dream and eraser at the end of T in this order, to obtain S = T.

Sample Input 3

dreamerer

Sample Output 3

NO

Solution in OCaml (4.02.3) (judge verdict: Accepted)


let memoize n f =
  let dp = Hashtbl.create n in
  let rec get x =
    try Hashtbl.find dp x with
    | Not_found ->
        let result = f get x in
        Hashtbl.add dp x result;
        result in get

module RegExp : sig
  type t
  val empty_set : t
  val empty_str : t
  val char : Char.t -> t
  val app : t -> t -> t
  val union : t -> t -> t
  val star : t -> t

  val matches : t -> Char.t list -> bool
end = struct
  type t =
    { sort : sort;
      is_nullable : bool;
      derive : char -> t }
  and sort =
    | EmptySet
    | EmptyStr
    | Star
    | Other

  let rec empty_set =
    { sort = EmptySet;
      is_nullable = false;
      derive = fun _ -> empty_set }

  let empty_str =
    { sort = EmptyStr;
      is_nullable = true;
      derive = fun _ -> empty_set }

  let char c =
    { sort = Other;
      is_nullable = false;
      derive = fun c' -> if c = c' then empty_str else empty_set }

  let rec union re1 re2 =
    match re1, re2 with
    | { sort = EmptySet }, _ -> re2
    | _, { sort = EmptySet } -> re1
    | _, _ ->
        { sort = Other;
          is_nullable = re1.is_nullable || re2.is_nullable;
          derive = memoize 64 @@ fun _ c -> union (re1.derive c) (re2.derive c) }

  let rec app re1 re2 =
    match re1, re2 with
    | { sort = EmptySet }, _ -> empty_set
    | _, { sort = EmptySet } -> empty_set
    | { sort = EmptyStr }, _ -> re2
    | _, { sort = EmptyStr } -> re1
    | _, _ ->
        { sort = Other;
          is_nullable = re1.is_nullable && re2.is_nullable;
          derive = memoize 64 @@ fun _ c ->
            union
              (if re1.is_nullable then re2.derive c else empty_set)
              (app (re1.derive c) re2) }

  let star = function
    | { sort = EmptySet }
    | { sort = EmptyStr } -> empty_str
    | { sort = Star } as re -> re
    | re ->
        let memo = Hashtbl.create 64 in
        let rec self =
          { sort = Star;
            is_nullable = true;
            derive = fun c ->
              try Hashtbl.find memo c with
              | Not_found ->
                  let d = app (re.derive c) self in
                  Hashtbl.add memo c d;
                  d } in self

  let matches re s =
    (List.fold_left (fun re -> re.derive) re s).is_nullable
end

let () =
  let s = read_line () in
  print_endline @@
  if
    let open RegExp in
    matches
      (* ((dream(er)?)|(erase(r)?))* *)
      (star @@
        union
          (app (char 'd') @@ app (char 'r') @@ app (char 'e') @@ app (char 'a') @@ app (char 'm') @@
            union empty_str @@ app (char 'e') @@ char 'r')
          (app (char 'e') @@ app (char 'r') @@ app (char 'a') @@ app (char 's') @@ app (char 'e') @@
            (union empty_str @@ char 'r'))) @@
      Array.to_list @@ Array.init (String.length s) (String.get s)
  then "YES"
  else "NO"