second

second.fs

type symbol =  Number of int | Plus of int * symbol | Minus of int * symbol | Noop of int * symbol

let merge (i1: int) (i2:int)  = (int)(sprintf "%d%d" i1 i2)

let rec flattenNoop ((i,s): (int*symbol)) =
    match s with
        | Number n -> Number (merge i n)
        | Plus (n,t) -> Plus (merge i n,t)
        | Minus (n,t) -> Minus (merge i n,t)
        | Noop (n,t) -> flattenNoop (merge i n,t)

let rec calc (s: symbol) : int =
        match s with
            | Number x -> x
            | Plus (x,s) -> x + calc(s)
            | Minus (x,s) -> x- calc(s)
            | Noop (x,s) -> calc(flattenNoop (x, s)) 
let rec genNumbers (s:int) (stopp: int) = seq {
    if s = stopp then yield Number s
    else for i in genNumbers (s+1) stopp do
             yield Plus (s,i) 
             yield Minus (s,i)
             yield Noop (s,i)
}

genNumbers 1 9 |> Seq.filter (fun x -> 100 = calc x)|> Seq.distinct|> Seq.toList