open System

​

let (?>) x y = Char.GetNumericValue(x) > Char.GetNumericValue(y)

let rec check (list:string) (j:int) (len:int) =

    if j < len then

        if list.[j] ?> list.[j+len] then false

        else check list (j+1) (len / 2)

    else true

let rec func (input:string) (length:int) =

    if check input 0 length then 

        if length <> 1 then

            let s1 = input.Remove(length, length)

            let s2 = input.Remove(0, length)

            let nlng = length / 2

            (func s1 nlng) && (func s2 nlng)

        else true

    else false

let str = "00101111"

func str (str.Length/2)

|> printfn "%A"