题解 | #添加逗号#

用字符串的形式从后往前每三个插入一个','

```#include <iostream>
#include <cstring>
using namespace std;
int main()
{
    string s,s1;
    int l,f = 0;
    cin>>s;
    l = s.length();
    for(int i = l-1;i>=0;i--)
    {
        if(f==3)
        {
            f = 0;
            s1=','+s1;
        }
            s1 = s[i]+s1;
            f++;
    }
    cout<<s1<<endl;
    return 0;
}