let chiffres = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";;
let chiffre v =
  if 0 <= v && v < String.length chiffres then
    String.sub chiffres v 1
  else
    "?(" ^ string_of_int v ^ ")"
;;

let rec ecriture b n =
  if n = 0 then
    ""
  else
    let alpha = n mod b in
    let dernier = chiffre alpha in
    let n' = n / b in
    let reste = ecriture b n' in
    reste ^ dernier
;;

let rec ecriture' b n =
  if n = 0 then
    ""
  else
    (ecriture' b (n / b)) ^ (chiffre (n mod b))
;;

let ecrit b n =
  if n < 0 then
    failwith "Pas de nombre negatif"
  else if n = 0 then
    print_endline "0"
  else
    print_endline (ecriture b n)
;;

let ecrit_16 = ecrit 16;;

ecrit_16 65535;;
ecrit 37 36;;
ecrit 37 65535;;