class Solution {
    public String addStrings(String num1, String num2) {
        StringBuilder sb = new StringBuilder();

        int carry = 0;
        for (int i = num1.length() - 1, j = num2.length() - 1; i >= 0 || j >= 0 || carry > 0; i--, j--)
        {
            int digit1;
            int digit2;
            if (i < 0)
            {
                digit1 = 0;
            }
            else
            {
                digit1 = num1.charAt(i) - '0';
            }
            if (j < 0)
            {
                digit2 = 0;
            }
            else
            {
                digit2 = num2.charAt(j) - '0';
            }

            int sum = digit1 + digit2 + carry;
            carry = sum / 10;
            sum = sum % 10;

            sb.append(sum);
        }

        return sb.reverse().toString();
    }
}