(* insertion sorting via binary search trees with more efficient flattening *)

type 'a tree = Leaf
             | Node of 'a * 'a tree * 'a tree

let rec insert x tr =
      match tr with
            Leaf              -> Node(x, Leaf, Leaf)
          | Node(y, tr1, tr2) ->
              match compare x y with
                    -1 -> Node(y, insert x tr1, tr2)
                  | 0  -> tr
                  | _  -> Node(y, tr1, insert x tr2)

let rec insertList xs tr =
      match xs with
            []      -> tr
          | x :: xs -> insertList xs (insert x tr)

(* optimal in terms of ::'s *)

let rec flattenOnto tr xs =
      match tr with
            Leaf              -> xs
          | Node(x, tr1, tr2) ->
              flattenOnto tr1 (x :: flattenOnto tr2 xs)
                                    
let flatten tr = flattenOnto tr []

let sort xs = flatten(insertList xs Leaf)

(* try: sort [3;1;2;5;3];; *)