题解 | 添加逗号

import java.util.Scanner;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        Long N = in.nextLong();
        String st = Long.toString(N);
        StringBuilder string = new StringBuilder(st);
        int n = st.length() / 3;
        int m = st.length() % 3;
        if(m == 0){
            int count = 0;
        for(int i = 1; i < n; i++){
            string.insert(3*i + count,',');
            count++;
        }
        System.out.print(string);
        }else if(m != 0 && n != 0){
            int count = 0;
        for(int i = 0; i < n; i++){
            string.insert(3*i + m + count,',');
            count++;
        }
        System.out.print(string);
        }else{
            System.out.print(string);
        }
        
    }
}