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-; index > ; index -= )
       str.insert(index, ",");
     cout<<str<<endl;
     return ;
 }