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.

Example:

Input:  n = 2
Output: ["11","69","88","96"]

思路：不知道和回溯有啥關係？感覺是去要去加。
cur = ans;
然後根據cur，給ans里加東西
這是用iterative的方法去加，還蠻特殊的

for (;n > 1; n -= 2) {
            cur 
 
= ans;
            ans = new ArrayList<String>();
            for (String i : cur) {
                if (n > 3) ans.add('0' + i + '0');

如果長度 > 3, 就可以有'010'這種被0夾的數字

class Solution {
    public List<String> findStrobogrammatic(int n) {
        List<String> cur, ans;
        ans 
 
= new ArrayList<String>((n & 1) == 0 ? Arrays.asList("") : Arrays.asList("0", "1", "8"));
        if (n < 2) return ans;
        
        for (; n > 1; n -= 2) {
            cur = ans;
            ans = new ArrayList<String>();
            
            for (String currentString : cur) {
                
 
if (n > 3)
                    ans.add('0' + currentString + '0');
                ans.add('1' + currentString + '1');
                ans.add('6' + currentString + '9');
                ans.add('9' + currentString + '6');
                ans.add('8' + currentString + '8');
            }

        }
        return ans;
    }
}