follow up: 换成八进制

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