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 s = String.valueOf(n);

        int cnt = (s.length() - 1) / 3;

        if (cnt == 0) {
            System.out.print(s);
            return;
        }

        StringBuilder sb = new StringBuilder();

        int first = s.length() % 3;
        if (first == 0) {
            first = 3;
        }

        sb.append(s.substring(0, first));
        for (int i = first; i < s.length(); i += 3) {
            sb.append(",");
            sb.append(s.substring(i, Math.min(i + 3, s.length())));
        }

        System.out.print(sb.toString());
    }
}