高精度函数运行不了
string add (string a, string b){
string c;
int maxlen = max (a.length (), b.length ());
reverse (a.begin (), a.end ());
reverse (b.begin (), b.end ());
for (int i = 0; i < maxlen; i++){
int k = (a[i] - '0') + (b[i] - '0');
c[i] = k % 10;
c[i - 1] = k / 10;
}
return c;
}