【LeetCode 67_字符串_算术运算】Add Binary

 string addBinary(string a, string b)
 {
     int alen = a.size();
     int blen = b.size();
     if (alen == )
         return b;
     else if (blen == )
         return a;
 
     int i = alen - ;
     int j = blen - ;
     int carry = ;
     string res;
     while (i >=  || j >=  || carry > ) {
         if (i >= )
             carry += a[i--] - '';
         if (j >= )
             carry += b[j--] - '';
         res = (char)(carry %  + '') + res;        //res不用逆序处理了
         carry = carry / ;
     }
     return res;
 }