step 1 : Long타입변수.toString()
step 2 :String타입변수.toCharArray()
~~ 뭔가 처리를 한다 ~~
step 3 : String(CharArray타입변수).toLong()
class Solution {
fun solution(n: Long): Long {
var answer: Long = 0
var nString = n.toString().toCharArray()
answer = String(nString.sortedArrayDescending()).toLong()
return answer
}
}