let rec printInOrder tree key codeList =
    match tree with
    |Node (data, left, right) ->
        printInOrder left (key @ [0]) codeList
        printfn "Symbol '%s' %A" data key
        let codeList' = codeList @ [(data, key)]
        printInOrder right (key @ [1]) codeList'
    |Empty -> codeList