题解 | #动态字符串# StringBuilder使用

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String str = scanner.next();

        //write your code here......
        // int n = str.length()/3;
        // StringBuilder str2 = new StringBuilder("");

        // for(int i = 0; i < str.length(); i++)
        // {
        //     str2.append(str.charAt(str.length() - i -1));
        //     if( (i+1)%3 == 0)
        //     {
        //         str2.append(",");
        //     }
        // }
        // System.out.println(str2.reverse());

        StringBuilder str2 = new StringBuilder(str);
        for(int i = str2.length()-3; i > 0; i-=3)
        {
            str2.insert(i,",");
        }
        System.out.println(str2);

    }
}

StringBuilder的灵活使用