past3_f.ml

let n = read_int ()
let n2 = n / 2
(* 文字列をcharのlistにする *)
let explode s = List.init (String.length s) (String.get s)
let is_even = (if n mod 2 = 1 then true else false)
let a = Array.init n @@ fun _ -> read_line () |> explode
let rec main_loop i result =
  if i >= n2 then result else
  let rec loop lst =
    match lst with
    | [] -> '0'
    | first :: rest -> if List.mem first a.(n-1-i) then first else loop rest
  in let p = loop a.(i) in
  if p = '0' then [] else
  main_loop (i+1) (p :: result)
(* charのlistをstringに *)
let string_of_chars chars = 
  let buf = Buffer.create 16 in
  List.iter (Buffer.add_char buf) chars;
  Buffer.contents buf
let r = main_loop 0 []
let _ = (
        if n = 1 then string_of_chars [List.hd a.(0)] else
        if r = [] then "-1" else
         if is_even then string_of_chars ((List.rev r) @ [List.hd a.(n2)] @ r) else
        string_of_chars ((List.rev r) @ r)) |> print_endline