Leetcode: Add Strings

Given two non-negative numbers 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.

public class Solution {
    public String addStrings(String num1, String num2) {
        StringBuffer res = new StringBuffer();
        int i = num1.length()-1;
        int j = num2.length()-1;
        int carry = 0;
        while (i>=0 || j>=0 || carry!=0) {
            int sum = 0;
            if (i >= 0) {
                sum += (int)(num1.charAt(i) - '0');
                i--;
            }
            if (j >= 0) {
                sum += (int)(num2.charAt(j) - '0');
                j--;
            }
            if (carry != 0) {
                sum += carry;
            }
            int digit = sum % 10;
            carry = sum / 10;
            res.insert(0, digit);
        }
        return res.toString();
    }
}