LeetCode - Add Strings

Given two non-negative integers num1 and num2 represented as string, return the sum of num1 and num2.

Note:

The length of both num1 and num2 is < 5100.
Both num1 and num2 contains only digits 0-9.
Both num1 and num2 does not contain any leading zero.
You must not use any built-in BigInteger library or convert the inputs to integer directly.

class Solution {
    public String addStrings(String num1, String num2) {
        StringBuilder res = new StringBuilder();
        int i = num1.length()-1;
        int j = num2.length()-1;
        int carry = 0;
        while (i >= 0 || j >= 0) {
            int a = i >= 0 ? num1.charAt(i) - '0' : 0;
            int b = j >= 0 ? num2.charAt(j) - '0' : 0;
            int sum = a + b + carry;
            res.insert(0, sum % 10);
            carry = sum / 10;
            j--;
            i--;
        }
        return carry == 1 ? res.insert(0, "1").toString() : res.toString();
    }
}