Flatten:-
It provides flat access to nested collection elements .
flatten () is a collection of collections, for example, array of array, a List of Sets or List of List. The function returns a single collection of all the elements of the nested collections.
val numberSets = listOf(setOf(11, 22, 33), setOf(44, 55, 66), setOf(77,88))
println(numberSets.flatten())
```
output:-
`[11, 22, 33, 44, 55, 66, 77, 88]`
**joinToString()** builds a single String from the collection elements based on the provided arguments.
```
val numbers = listOf("blue", "red", "orange", "pink")
println(numbers)
println(numbers.joinToString())
```
output
`[blue, red, orange, pink]`
`blue, red, orange, pink`