def encode(tree: CodeTree)(text: List[Char]): List[Bit] = {
    def encodeChar(partial: CodeTree)(char: Char)(encoded: List[Bit]): List[Bit] = partial match {
      case Leaf(_, _) => encoded
      case Fork(left, right, c, w) =>
        if (chars(left).contains(char)) 0 :: encodeChar(left)(char)(encoded)
        else 1 :: encodeChar(right)(char)(encoded)
    }

    text.flatMap(c => encodeChar(tree)(c)(List[Bit]()))
  }