public List<List<String>> partition(String s) {
        // write your code here
       List<List<String>> res = new ArrayList<>();
        if (s == null) return res;
        dfs(res, new ArrayList<String>(), 0, s);
        return res;
    }
    public static void dfs(List<List<String>> res, ArrayList<String> list, int start, String s) {
      if (start == s.length()) {
          res.add(new ArrayList<String>(list));
          return;
      }

        for (int i = start; i < s.length(); i++) {
            String sub = s.substring(start, i + 1);
            if (!isPalindrome(sub)) {
                continue;
            }
            list.add(sub);
            dfs(res, list, i + 1, s);
            list.remove(list.size() - 1);
        }
    }
    public static boolean isPalindrome(String s) {
        if (s == null || s.length() == 0) return false;
        if (s.length() == 1) return true;
        int start = 0;
        int end = s.length() - 1;
        while (start < end) {
            if (s.charAt(start) != s.charAt(end)){
                return false;
            }
            start++;
            end--;
        }
        return true;
    }