C++ 一串数字三位一节，用逗号隔开表示

#include <iostream>
#include <string>
#include <sstream>
using namespace std;
string LongToString(long l)
{
    stringstream str;
    str<<l;
    return str.str();
}
int main()
{
    long l;
    cin>>l;
    string str = LongToString(l);
    size_t len = str.length();
    for(int index =(int) len-3; index > 0; index -= 3)
      str.insert(index, ",");
    cout<<str<<endl;
    return 0;
}