private String aftDo(String[] list) {
        int val =0;
        int length  = list.length;
        if(length == 0)
            return null;
        else
            list[length -1] ="";
        for(int j = list[0].length()-1; j>=0;j--){
            val = (val / 10) ;
            for(int i=0; i< list.length-1;i++ ){
                val += list[i].charAt(j)-'0';
            }
            list[list.length-1]= val% 10 + list[list.length-1];
        }
        // delete pre 0
        String result = list[length-1];
        for(int i =0;i<result.length();){
            if(result.length() != 1 && result.charAt(0) == '0')
                result= result.substring(1);
            else
                break;
        }
        return result;
    }