List all combinations of picking n elements.

combinations.kt

fun <T> Collection<T>.combinations(n: Int): List<List<T>> {
    return when {
        n < 0 -> throw Error("combinations size cannot be negative")
        n > size -> emptyList()
        n == 1 -> map { mutableListOf(it) }
        else -> foldIndexed<T, List<List<T>>>(mutableListOf()) { index, all, first ->
            all + drop(index + 1)
                    .combinations(n - 1)
                    .map { it + first }
        }
    }
}

val list = listOf(1, 2, 3, 4)
list.combinations(2) // => [[2, 1], [3, 1], [4, 1], [3, 2], [4, 2], [4, 3]]
list.combinations(3) // => [[3, 2, 1], [4, 2, 1], [4, 3, 1], [4, 3, 2]]