题解 | #动态字符串#

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String str = scanner.next();
        int i;
        
        //使用可变字符串StringBuilder,参数为静态的字符串str
        StringBuilder str_test=new StringBuilder(str);
        for(i=str.length()-3;i>=0;i-=3){
            //插入","
            str_test.insert(i,",");
        }
        System.out.println(str_test);
        //write your code here......
    }
}