Scanf.scanf "%d" (fun n ->
    let pos = Array.make (n + 1) 0 in
    let module S = Set.Make (struct type t = int * int let compare = compare end) in
    let rec loop i set =
        if i = n then set else
            loop (i + 1) (Scanf.scanf " %d" (fun v ->
                pos.(v) <- i;
                S.add (i, v) set))
    in
    let set = loop 0 S.empty in
    let rec loop rest mi q set =
        if rest = 0 then List.rev q else
            if pos.(mi) < 0 then loop rest (mi + 1) q set else
            let p = pos.(mi) in
            let _, _, r = S.split (p, mi - 1) set in
            let c0, v0 = S.min_elt r in
            let r = S.remove (c0, v0) r in
            if S.is_empty r then loop rest (mi + 1) q set else
                let c1, v1 = S.min_elt r in
                let set = S.remove (c0, v0) set in
                let set = S.remove (c1, v1) set in
                let () = pos.(v0) <- -1 in
                let () = pos.(v1) <- -1 in
                loop (rest - 2) mi (v1 :: v0 :: q) set
    in
    let q = loop n 1 [] set in
    List.iter (Printf.printf "%d ") q;
    print_newline ()
)