思路

public String addBinary(String a, String b) {
        StringBuilder ans = new StringBuilder();
        int carry = 0;
        for(int i=a.length()-1,j=b.length()-1;i>=0||j>=0;i--,j--){
            int sum = carry;
            sum += i>=0?a.charAt(i)-'0':0;
            sum += j>=0?b.charAt(j)-'0':0;
            ans.append(sum%2);
            carry=sum/2;
        }
        if(carry==1)
            ans.append(carry);
        return ans.reverse().toString();
}