class Factorial1 {
    @tailrec    // compilation ERROR
    def factorial(n: Int): Int = {
        if (n <= 1) 1
        else n * factorial(n - 1)
    }
}

//----------------------------------------

import scala.annotation.tailrec

class Factorial2 {
    def factorial(n: Int): Int = {

        @tailrec    // OK
        def factorialAcc(acc: Int, n: Int): Int = {
            if (n <= 1) acc
            else factorialAcc(n * acc, n - 1)
        }

        factorialAcc(1, n)
    }
}