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.

Example:

Input: low = "50", high = "100"
Output: 3 
Explanation: 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.

思路: 既然是follow up，那麼就承接 Strobogrammatic Number II  的代碼；利用II的代碼，生成size爲low 和high的數字，然後判斷一下是否在範圍內即可，注意用Long來計算，防止爆範圍；

class Solution {
    public int strobogrammaticInRange(String low, String high) {
        int lowlen = low.length();
        int highlen = high.length();
        int count = 0;
        long lownum = Long.parseLong(low);
        long highnum = Long.parseLong(high);
        for(long i = lowlen; i <= highlen; i++) {
            List<String> list = helper(i, i);
            for(String str: list) {
                long temp = Long.parseLong(str);
                if(lownum <= temp && temp <= highnum) {
                    count++;
                }
            }
        }
        return count;
    }
    
    private List<String> helper(long curlen, long n) {
        if(curlen == 0) {
            return new ArrayList<String>(Arrays.asList(""));
        }
        if(curlen == 1) {
            return new ArrayList<String>(Arrays.asList("0", "1", "8"));
        }
        //假設當前已經做好了 n - 2的size的數字，只需最後一層；
        List<String> list = helper(curlen - 2, n);
        List<String> res = new ArrayList<String>();
        for(String str: list) {
            // 這個很巧妙，也就是最最外面的一層，不能加0；裏面都可以加0；
            if(curlen != n) {
                res.add("0" + str + "0");
            }
            res.add("1" + str + "1");
            res.add("6" + str + "9");
            res.add("9" + str + "6");
            res.add("8" + str + "8");
        }
        return res;
    }
}