open System.Collections.Generic

​

let canConstruct (ransomNote : string) (magazine : string) =

  let letters = new Dictionary<char, int>()

  ransomNote 

  |> Seq.iter (fun char ->

    match letters.TryGetValue char with

    | true, n -> letters.[char] <- n+1

    | _       -> letters.[char] <- 1 

  )

​

  magazine

  |> Seq.iter (fun char -> 

    match letters.TryGetValue char with

    | true, n when n > 1 -> letters.[char] <- n-1

    | true, _ -> letters.Remove char |> ignore

    | _       -> ())

​

  letters.Count = 0

​

printfn "%A" <| canConstruct "aa" "abba"