```
string addStrings(string num1, string num2) {
string res;
int carry = 0;
int i = num1.size() - 1;
int j = num2.size() - 1;
while(i >= 0 || j >= 0)
{
int now = (i >= 0 ? num1[i] : '0') + (j >= 0 ? num2[j] : '0') - '0'*2 + carry;
res.push_back('0' + now%10);
carry = now/10;
--i,--j;
}
if(carry)
res.push_back('1');
reverse(res.begin(), res.end());
return res;
}
```