let string_of_list to_s list = 
  "[" ^ String.concat ", " (List.map to_s list) ^ "]"
let print_list to_s list = 
  print_endline (string_of_list to_s list)
let isodd n = n mod 2 <> 0
let f ns = 
  let ms = List.sort compare (List.filter isodd ns) in
  let rec aux acc = function
    | [], _ -> acc
    | n :: ns, m :: ms when isodd n -> aux (m :: acc) (ns, ms)
    | n :: ns, ms -> aux (n :: acc) (ns, ms)
  in
  List.rev (aux [] (ns, ms))
let () = 
  let p = print_list string_of_int in
  p (f [6; 5; 4; 3; 2; 1]);
  p (f [6; 5; 3; 1])
 

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