415. Add Strings

public class Solution {
   public String addStrings(String num1, String num2) {
         StringBuilder result = new StringBuilder();
        char[] n1 = num1.toCharArray();
        char[] n2 = num2.toCharArray();

        int size1 = n1.length - 1;
        int size2 = n2.length - 1;
        int mindSum = 0;
        while (size1 >= 0 && size2 >= 0){
            int sum = n2[size2--] + n1[size1--] + mindSum - 96;
            mindSum = sum > 9 ? 1 : 0;
            result.insert(0, sum % 10);
        }
        while (size2 >= 0){
            int sum = n2[size2--] + mindSum - 48;
            mindSum = sum > 9 ? 1 : 0;
            result.insert(0, sum % 10);
        }
        while (size1 >= 0){
            int sum = n1[size1--] + mindSum - 48;
            mindSum = sum > 9 ? 1 : 0;
            result.insert(0, sum % 10);
        }
        if(mindSum > 0){
            result.insert(0, 1);
        }
        return result.toString();
    }
}