参考答案：

#include<iostream>
#include <cctype>
#include<string>
#include<algorithm>
using namespace std;
int main()
{
    string s, s2, s3;
    cin >> s;
    int i = 0, j = 0;
    for (i = s.length() - 1; i >= 0; --i)
    {
        if (j == 3)
        {
            j = 1;
            s2 += ',';
            s2 += s[i];
        }
        else 
        {
            s2 += s[i];
            ++j;
        }
    }
    for (i = s2.length() - 1; i >= 0; --i) cout << s2[i];
    return 0;
}