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public class Solution {
    public List<String> findStrobogrammatic(int n) {
        List<String> one = Arrays.asList("0", "1", "8"), two = Arrays.asList(""), r = two;
        if(n%2 == 1)
            r = one;
        for(int i=(n%2)+2; i<=n; i+=2){
            List<String> newList = new ArrayList<>();
            for(String str : r){
                if(i != n)
                    newList.add("0" + str + "0");
                newList.add("1" + str + "1");
                newList.add("6" + str + "9");
                newList.add("8" + str + "8");
                newList.add("9" + str + "6");
            }
            r = newList;
        }
        return r;   
    }

}

public class Solution {
    char[][] pairs = new char[][]{{'0', '0'}, {'1', '1'}, {'8', '8'}, {'6', '9'}, {'9', '6'}};
    public List<String> findStrobogrammatic(int n) {
        List<String> res = new ArrayList<>();
        char[] build = new char[n];
        dfs(res, build, 0, n - 1);
        return res;
    }
    private void dfs(List<String> res, char[] build, int left, int right){
        if(left > right){
            String s = new String(build);
            res.add(s);
            return;
        }
        for(int i = 0; i < pairs.length; i++){
            build[left] = pairs[i][0];
            build[right] = pairs[i][1];
            if(build.length > 1 && build[0] == '0') continue;
            if(left == right){
                if(i == 3 || i == 4) continue;
            }
            dfs(res, build, left + 1, right - 1);
        }
    }
}

public class Solution {
    private char[][] stroNums = {{'0', '0'}, {'1', '1'}, {'8','8'}, {'6', '9'}, {'9', '6'}};
    
    public List<String> findStrobogrammatic(int n) {
        List<String> res = new ArrayList<>();
        char[] ra = new char[n];
        if (n % 2 == 0) {
            setResults(res, ra, n/2 - 1);
        } else {
            ra[n/2] = '1';
            setResults(res, ra, n/2 - 1);
            ra[n/2] = '8';
            setResults(res, ra, n/2 - 1);
            ra[n/2] = '0';
            setResults(res, ra, n/2 - 1);
        }
        
        return res;
    }
    
    private void setResults(List<String> res, char[] ra, int pos) {
        if (pos < 0) {
            res.add(new String(ra));
            return;
        }
        
        for (char[] sc : stroNums) {
            if (pos == 0 && sc[0] == '0') continue;
            
            ra[pos] = sc[0];
            ra[ra.length - 1 - pos] = sc[1];
            setResults(res, ra, pos - 1);
        }
    }
}

public class Solution {
    private List<String> generate(int left, int amount) {
        if (left == 0) return new ArrayList<String>(Arrays.asList(""));
        if (left == 1) return new ArrayList<String>(Arrays.asList("0", "1", "8"));
        
        List<String> mid = generate(left - 2, amount);
        
        List<String> res = new ArrayList<>();
        for (String s : mid) {
            if (left != amount) {
                res.add("0" + s + "0");
            }
            res.add("1" + s + "1");
            res.add("6" + s + "9");
            res.add("8" + s + "8");
            res.add("9" + s + "6");
        }
        return res;
    }
    public List<String> findStrobogrammatic(int n) {
        return generate(n, n);
    }
}