I have a `Dataset` like the following

```
monthYear    code
201601       11
201601       12
201601       12
201601       10
201602       null
201602       21
201602       21
201602       21
201603       null

```

When `code` is `null` I want to replace that with the `code` that appeared the most during the last month. For the above example, the first `null` will get replaced by `12` and the second one with `21`.

So the result would be the following.

```
monthYear    code
201601       11
201601       12
201601       12
201601       10
201602       12
201602       21
201602       21
201602       21
201603       21

```

How can I achieve this?