LeetCode-Strobogrammatic Number II

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

Find all strobogrammatic numbers that are of length = n.

For example,
Given n = 2, return ["11","69","88","96"].

Solution:

public class Solution {
    static char[] nums1 = new char[] { '0', '1', '8', '6', '9' };
    static char[] nums2 = new char[] { '0', '1', '8', '9', '6' };

    public List<String> findStrobogrammatic(int n) {
        List<String> res = new ArrayList<String>();
        StringBuilder builder = new StringBuilder();
        findStrobogrammaticRecur(n, builder, 1, res);

        return res;
    }

    public void findStrobogrammaticRecur(int n, StringBuilder builder, int curLevel, List<String> res) {
        if (curLevel > n / 2) {
            if (n % 2 == 1) {
                // add one more char into front, no '6' or '9'
                for (int i = 0; i < 3; i++) {
                    builder.insert(n / 2, nums1[i]);
                    res.add(builder.toString());
                    builder.deleteCharAt(n / 2);
                }
            } else
                res.add(builder.toString());
            return;
        }

        // if curLevel is n/2, we should skip '0'.
        int start = (curLevel == n/2) ? 1 : 0;
        for (int i = start; i < 5; i++) {
            builder.insert(0, nums1[i]);
            builder.append(nums2[i]);
            findStrobogrammaticRecur(n, builder, curLevel + 1, res);
            builder.deleteCharAt(0);
            builder.deleteCharAt(builder.length() - 1);
        }
    }
}