let rec partition f = function
  | [] -> [], []
  | x::xs ->
    let ys, zs = partition f xs in
    if f x 
      then x::ys, zs
      else ys, x::zs

let rec qsort = function
  | [] -> []
  | p::xs -> 
    let ys, zs = partition (fun x -> x <= p) xs in
    qsort ys @ [p] @ qsort zs

let li = [5;1;4;2;3;0]
let sorted = qsort li
let odds_evens = partition (fun x -> x % 2 = 1) sorted
printfn "%A" sorted
printfn "%A" odds_evens