string addStrings(string num1, string num2) {

string result = "";

int carry = 0, sum = 0;

int m = (int)num1.length(), n = (int)num2.length();

int i = m - 1, j = n - 1;

while(i >= 0 or j >= 0 or carry) {

int digit1 = (i >= 0 ? num1[i] - '0' : 0);

int digit2 = (j >= 0 ? num2[j] - '0' : 0);

sum = digit1 + digit2 + carry;

result += to_string(sum % 10);

carry = sum / 10;

--i, --j;

}

reverse(result.begin(), result.end());

return result;

}