Strobogrammatic Number III

A strobogrammatic number is a number that looks the same when rotated 180 degrees (looked at upside down).

Write a function to count the total strobogrammatic numbers that exist in the range of low <= num <= high.

For example,
Given low = "50", high = "100", return 3. Because 69, 88, and 96 are three strobogrammatic numbers.

Note:
Because the range might be a large number, the low and high numbers are represented as string.

public class Solution {

    public int strobogrammaticInRange(String low, String high) {
        if (low.length() > high.length() || (low.length() == high.length() && low.compareTo(high) > 0)) return 0;
        LinkedList<String> pre = new LinkedList<>();
        pre.add("");
        LinkedList<String> cur = new LinkedList<>();
        cur.add("1");
        cur.add("8");
        cur.add("0");
        int sl = low.length();
        int hl = high.length();
        int n = 1;
        int res = 0;
        while (true) {
            if (n >= sl && n <= hl) {
                for (String s : cur) {
                    if (s.charAt(0) == '0' && s.length() != 1) {
                        continue;
                    }
                    if (n == sl && s.compareTo(low) < 0) {
                        continue;
                    }
                    if (n == hl && s.compareTo(high) > 0) {
                        continue;
                    }
                    res++;
                }
            }
            if (n == hl) {
                break;
            }
            LinkedList<String> next = getNextStro(pre);
            pre = cur;
            cur = next;
            n++;
        }
        return res;
    }
   
    private LinkedList<String> getNextStro(LinkedList<String> pre) {
        int count = pre.size();
        while (count-- > 0) {
            String s = pre.poll();
            pre.addLast("0" + s + "0");
            pre.addLast("1" + s + "1");
            pre.addLast("8" + s + "8");
            pre.addLast("6" + s + "9");
            pre.addLast("9" + s + "6");
        }
        return pre;
    }
}