Add Strings

1class Solution {
2    public String addStrings(String num1, String num2) {
3        StringBuilder sb = new StringBuilder();
4
5        int carry = 0;
6        for (int i = num1.length() - 1, j = num2.length() - 1; i >= 0 || j >= 0 || carry > 0; i--, j--)
7        {
8            int digit1;
9            int digit2;
10            if (i < 0)
11            {
12                digit1 = 0;
13            }
14            else
15            {
16                digit1 = num1.charAt(i) - '0';
17            }
18            if (j < 0)
19            {
20                digit2 = 0;
21            }
22            else
23            {
24                digit2 = num2.charAt(j) - '0';
25            }
26
27            int sum = digit1 + digit2 + carry;
28            carry = sum / 10;
29            sum = sum % 10;
30
31            sb.append(sum);
32        }
33
34        return sb.reverse().toString();
35    }
36}