I'm implementing a function that takes a `[[Int]]`, and return a `[String]`, it needs to fill the empty place in each sublist with `_`s, which index is the complement of the input list, and generate a string from the list, the length of each string is the same and is the (maximum of the input number + 1).

For example, if the input is `[[1, 2] [0, 1, 2, 3] [1, 3] [0, 2, 3]]`, the output would be `["_12_", "0123", "_1_3", "0_23"]`

I tried my best to do this, and don't know how to insert empty space into the missing part.

```
getString :: [[Int]] -> [String]
getString x = concat. show. x insert _ 
   where insert _ [] ys = ys

```