//String 1234...递增 求str[m]
    //1-9  10-99 100-999 .....
    //位数 1*9 2*90 3*900 .....
    public static Character getM(int m){
        int count = 1;
        int flag = 9;
        int res = m + 1; //下标从0开始 所以加1
        while(res > 0){
            res -= count * flag;
            count ++;
            flag *= 10;
        }
        count --;
        flag /= 10;
        res += count * flag;
        int shang = (res - 1) / count;
        int yu = (res - 1) % count;
        res = (count - 1) * (flag / 10) + 1 + shang;
        String str = String.valueOf(res);
        return str.charAt(yu);
    }