67. Add Binary

class Solution {
    public String addBinary(String a, String b) {
        StringBuilder res = new StringBuilder();
        int aIdx = a.length()-1;
        int bIdx = b.length()-1;
        
        int carry = 0;
        while (aIdx >= 0 || bIdx >= 0) {
            int aNum = aIdx >= 0 ? a.charAt(aIdx) - '0' : 0;
            int bNum = bIdx >= 0 ? b.charAt(bIdx) - '0' : 0;
            int sum = aNum + bNum + carry;
            carry = sum/2;
            res.append(sum%2);
            aIdx--;
            bIdx--;
        }
        if (carry == 1) {
            res.append(1);
        }
        return res.reverse().toString();
    }
}