So i have worked on it and found the solution for it

defmodule J do
  def join(list_of_lists) do
    join_single_row(list_of_lists, [])
    |> Enum.map(fn row -> List.flatten(row) end)
  end

  def join_single_row([], final), do: final

  def join_single_row([row | rest_of_rows], final) do
    length =
      Enum.map(row, fn [key, _value] -> key end)
      |> Enum.uniq()
      |> length()

    case length do
      1 ->
        join_single_row(rest_of_rows, final ++ [row])

      _ ->
        join_single_row(rest_of_rows, final)
    end
  end
end