递归的艺术

//普通递归
fun factoril(num:Int):BigInteger{
    if(num==0)return BigInteger.valueOf(1L)
    else{
        return BigInteger.valueOf(num.toLong()).times(factoril(num-1))
    }
}
fun main(args: Array<String>) {
    println(factoril(5))
}
//尾递归优化处理
class Result(var s:BigInteger=BigInteger.valueOf(1L))
tailrec fun factoril(num:Int,result: Result){
    if (num==0){
        result.s = result.s.times(BigInteger.valueOf(1L))
        //result.s = BigInteger.valueOf(1L)  变成常量了
    }else{
        result.s = result.s.times(BigInteger.valueOf(num.toLong()))
        factoril(num-1,result)
    }
}
fun main(args: Array<String>) {
    val result = Result()
    factoril(5,result)
    println(result.s)
}