leetcode 415 add strings

class  {
    public String addStrings(String num1, String num2) {
        int i=num1.length()-1;
        int j=num2.length()-1;
        int carry = 0;
        StringBuilder ans = new StringBuilder();
        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)%10;
            carry = (a+b+carry)/10;
            ans.insert(0, sum);
            j--;
            i--;
        }
        if (carry != 0)
            ans.insert(0, carry);
        return ans.toString();
    }
}