Add Strings

{4} Add Strings
Given two non-negative integers, num1 and num2 represented as string, return the sum of num1 and num2 as a string.
You must solve the problem without using any built-in library for handling large integers (such as BigInteger). You must also not convert the inputs to integers directly.
Example 1:

Input: num1 = "11", num2 = "123"
Output: "134"
Example 2:

Input: num1 = "456", num2 = "77"
Output: "533"
Example 3:

Input: num1 = "0", num2 = "0"
Output: "0"

class Solution {
    public String addStrings(String num1, String num2) {
        String sb = new String();
       int i = num1.length()-1;
       int j = num2.length()-1;
       int cry = 0;
       while(i >= 0 || j >= 0)
       {
           if(i >= 0)
           cry += (int)(num1.charAt(i--) - '0');
           if(j >= 0)
           cry += (int)(num2.charAt(j--) - '0');
           int digit = cry%10;
           sb = digit + "" + sb;
           cry/= 10;
       }
       return (cry == 0)? sb : cry + "" + sb;
   }       
}