知识点：字符串处理、数字拆分、模拟

法1）字符串处理 + 模拟

#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;
const int N = 30;
int a, b, ans[N], id;
int main() {
    cin >> a >> b;
    a += b;
    if(a < 0) cout << '-' ;
    string s = to_string(abs(a));
    for (int i = 0; i < s.size(); i ++ ) {
        cout << s[i] ;
        if(s[i + 1] && (s.size() - i) % 3 == 1) cout << ',';
    }
    return 0;
}

法2）数字拆分处理 + 模拟

#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;
const int N = 30;
int a, b, ans[N], id;
int main() {
    cin >> a >> b;
    a += b;
    if(a < 0) cout << '-' ;
    a = abs(a);
    if(!a) cout << 0 << endl;
    while (a) {
        ans[id ++ ] = a % 10;
        a /= 10;
    }
    for (int i = id - 1; i >= 0; i -- ) {
        cout << ans[i];
        if(i && i % 3 == 0) cout << ',' ;
    }
    return 0;
}