题解 | 动态字符串

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......
        StringBuilder sb = new StringBuilder(str);
        int i = sb.length() % 3;
        for (  ; i < sb.length() ; i+=4 ) {
            sb = sb.replace(i,i+1, "," + sb.charAt(i));
        }
        System.out.println(sb);

    }
}