DouKaku?

let trade arr i j =
  let get,set = String.get,String.set in
  let carry x y = set arr i y; set arr j x in
  carry (get arr i) (get arr j);;

module MBstr = struct
  let byte = ref 3 

  let char_size = function
    | '\000'..'\127' -> 1
    | _ -> !byte;;
  
  let rev s =
    let len = String.length s in
    let res = String.create len in
    let rec loop i j =
      if i>=len then res else (
        let size = char_size s.[i] in
        String.blit s i res (j-size) size;
        loop (i+size) (j-size)
      )
    in loop 0 len;;
  
  let rev_brackets op cl s =
    let len = String.length s in
    let rec loop i acc =
      if i>=len then s else
      loop (char_size s.[i] + i) (
        if String.contains op s.[i] then (i::acc) else (
          try (if op.[String.index cl s.[i]] = s.[List.hd acc]
               then trade s (List.hd acc) i);  List.tl acc
          with Not_found | Failure _ -> acc
      ))
    in loop 0 [];;
end;;

let reverse_string2 s =
  print_endline (MBstr.rev_brackets ")]}" "([{" (MBstr.rev s));;