class Solution {
public:
    /**
     * @param num1 a non-negative integers
     * @param num2 a non-negative integers
     * @return return sum of num1 and num2
     */
    string addStrings(string& num1, string& num2) {
        // Write your code here
        int i = num1.size() - 1;
        int j = num2.size() - 1;
        int carry = 0;
        string result = "" ;
        while (i >= 0 || j >= 0) {
            if (i>=0) {
                carry += num1[i--] - '0';
            }
            if (j>=0) {
                carry += num2[j--] - '0';
            }
            result = to_string(carry % 10) + result;
            carry /= 10;
        }
        return carry ? "1" + result : result;
    }
};