题解 | #添加逗号#

 #include <iostream>
#include <string>
#include <algorithm>
using namespace std;

int main()
{
    string str;
    cin >> str;
    reverse(str.begin(),str.end());
    string retstr;
    size_t len = str.size();
    for(int i = 1;i <= len ;i++)
    {
        retstr += str[i-1];
        if(i%3 == 0 && i != len)
            retstr += ',';
    }
    reverse(retstr.begin(),retstr.end());
    cout << retstr << endl;
    return 0;
}