open System

let longest xs ys = if List.length xs > List.length ys then xs else ys

let rec lcs a b =
    match a, b with
    | [], _
    | _, []        -> []
    | x::xs, y::ys ->
        if x = y then
            x :: lcs xs ys
        else
            longest (lcs a ys) (lcs xs b)

[<EntryPoint>]
let main argv =
    let split (str:string) = List.init str.Length (fun i -> str.[i])
    printfn "%A" (String.Join("",
        (lcs (split "thisisatest") (split "testing123testing"))))
    0