You could do: ``` val input = listOf(1,2,3,4,-1,-2...

# getting-started

[uhe]

You could do:

val input = listOf(1,2,3,4,-1,-2,3)
val duplicate = input.asSequence()
            .mapIndexed { index, number -> number to input.subList(index, input.size).count { it == number } }
            .firstOrNull { it.second > 1 }
            ?.first
println(duplicate) // prints 3