Leetcode: 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.

Understand the problem:
The idea would be very close to the previous problem. So we find all the strobogrammatic numbers between the length of low and high. Note that when the n == low or n == high, we need to compare and make sure the strobogrammatic number we find is within the range.

Code (Java):

public class Solution {
    private int count = 0;
    private Map<Character, Character> map = new HashMap<>();
    
    public int strobogrammaticInRange(String low, String high) {
        if (low == null || low.length() == 0 || high == null || high.length() == 0) {
            return 0;
        }
        
        fillMap();
        
        for (int n = low.length(); n <= high.length(); n++) {
            char[] arr = new char[n];
            getStrobogrammaticNumbers(arr, 0, n - 1, low, high);
        }
        
        return count;
    }
    
    private void getStrobogrammaticNumbers(char[] arr, int start, int end, String low, String high) {
        if (start > end) {
            String s = new String(arr);
            if ((s.length() == 1 || s.charAt(0) != '0') && compare(low, s) && compare(s, high)) {
                count++;
            }
            return;
        }
            
        for (char c : map.keySet()) {
            arr[start] = c;
            arr[end] = map.get(c);
                
            if ((start < end) || (start == end && map.get(c) == c)) {
                getStrobogrammaticNumbers(arr, start + 1, end - 1, low, high);
            }
        }
    }
    
    // Return true if s1 <= s2
    private boolean compare(String s1, String s2) {
        if (s1.length() == s2.length()) {
            if (s1.compareTo(s2) <= 0) {
                return true;
            } else {
                return false;
            }
        }
        
        return true;
    }
    
    private void fillMap() {
        map.put('0', '0');
        map.put('1', '1');
        map.put('8', '8');
        map.put('6', '9');
        map.put('9', '6');
    }
}