上一次根本没人回啊啊啊啊啊啊 突发奇想字符串模拟的高精加，结果没有报错终端啥也不显示

string add (string a, string b){
    string c;
    int maxlen = max (a.length (), b.length ());
    for (int i = maxlen; i > 0; i--){
        int k = (a[i] - '0') + (b[i] - '0');
        c[i] = k % 10;
        c[i - 1] = k / 10;
    }
    return c;
}