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"].
[分析]
实现2参考https://leetcode.com/discuss/50412/ac-clean-java-solution，
看这么清爽的代码真是让人赏心悦目~
实现2的思路就是从中间往两端扩展，自己的实现1思路是从两端往中间扩展，注意两端不能为0.

public class Solution {
    // Method 2
    public List<String> findStrobogrammatic(int n) {
        return helper(n, n);
    }
    public List<String> helper(int n, int m) {
        if (n == 0) return new ArrayList<String>(Arrays.asList(""));
        if (n == 1) return new ArrayList<String>(Arrays.asList("0", "1", "8"));
        List<String> list = helper(n - 2, m);
        List<String> ret = new ArrayList<String>();
        for (String s : list) {
            if (n != m) ret.add("0" + s + "0");
            ret.add("1" + s + "1");
            ret.add("8" + s + "8");
            ret.add("6" + s + "9");
            ret.add("9" + s + "6");
        }
        return ret;
    }
    // Method 1
    char[] cand = {'0', '1', '8', '6', '9'};
    public List<String> findStrobogrammatic1(int n) {
        List<String> ret = new ArrayList<String>();
        if (n <= 0) return ret;
        recur(new char[n], 0, n - 1, ret);
        return ret;
    }
    public void recur(char[] item, int i, int j, List<String> ret) {
        if (i > j) {
            ret.add(new String(item));
            return;
        } else if (i == j) {
            for (int k = 0; k < 3; k++) {
                item[i] = cand[k];
                ret.add(new String(item));
            }
            return;
        } else {
            int start = i == 0 ? 1 : 0;
            for (int k = start; k < 5; k++) {
                if (k < 3) {
                    item[i] = item[j] = cand[k];
                } else if (k == 3) {
                    item[i] = '6'; item[j] = '9';
                } else {
                    item[i] = '9'; item[j] = '6';
                }
                recur(item, i + 1, j - 1, ret);
            }
        }
    }
}