415. Add Strings

class Solution {
    public String addStrings(String num1, String num2) {
        StringBuilder builder = new StringBuilder();
        int carry = 0;
        int i = num1.length() - 1;
        int j = num2.length() - 1;
        while (i >= 0 || j >= 0) {
            int iNum = (i >= 0) ? num1.charAt(i) - '0' : 0;
            int jNum = (j >= 0) ? num2.charAt(j) - '0' : 0;
            int sum = iNum + jNum + carry;
            carry = sum / 10;
            builder.append(sum % 10);
            i--;
            j--;
        }
        if (carry == 1) {
            builder.append(1);
        }
        return builder.reverse().toString();
    }
}