LeetCode - Strobogrammatic Number II

StrobogrammaticNumberII.java

// 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"].

public List<String> findStrobogrammatic(int n) {
    return helper(n, n);
}

private List<String> helper(int n, int len){
    List<String> res = new ArrayList<String>();
    if(n <= 0) {
        res.add("");
        return res;
    }
    if(n == 1){
        res.add("0");
        res.add("1");
        res.add("8");
        return res;
    }
    
    List<String> lst = helper(n - 2, len);
    for(String str : lst){
        res.add("1" + str + "1");
        res.add("6" + str + "9");
        res.add("8" + str + "8");
        res.add("9" + str + "6");
        if(len != n) res.add("0" + str + "0");
    }
    return res;
}