leetcode add binary

    public String addBinary(String a, String b) {        //     char resi = '0';        int lena = a.length();    int lenb = b.length();    int len = (lena > lenb ? lena : lenb) - 1;    int idxa = lena - 1, idxb = lenb - 1;            String ans = "";        while(idxa >= 0 || idxb >= 0) {    char ac = '0', bc = '0';    if(idxa >= 0) ac = a.charAt(idxa--);    if(idxb >= 0) bc = b.charAt(idxb--);        int digSum = (resi - '0') + (ac - '0') + (bc - '0');    if(digSum == 3) {    ans = "1" + ans;    resi = '1';    } else if(digSum == 2) {    ans = "0" + ans;    resi = '1';    } else if(digSum == 1) {    ans = '1' + ans;    resi = '0';    } else if(digSum == 0) {    ans = '0' + ans;    resi = '0';    }    }    if(resi == '1') return "1" + ans;        return ans;    }