题解 | 添加逗号

import java.util.Scanner

fun main(args: Array<String>) {
    val read = Scanner(System.`in`)
    val tar = read.nextLine()!!.toLong()
    println(formatWithCommas(tar))
}
fun formatWithCommas(n:Long):String{
    val numStr = n.toString()
    if(numStr.length<=3){
        return numStr
    }
    val result = StringBuilder()
    var count = 0
    for(i in numStr.length-1 downTo 0){
        if(count>0 && count%3 == 0){
            result.append(',')
        }
        result.append(numStr[i])
        count++
    }
    return result.reverse().toString()
}