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Would this be an acceptable solution?

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#1 
public static List<String> generateValidParentheses(int num) {
    LinkedList<String> result = new LinkedList<String>();
    
    if(num < 1) {
      return result;
    }

    result.add("()");    
    String tmp = "";
    Set<String> tr = new HashSet<>();
    for(int i = 1; i < num; i++) {
      int count = result.size();
      for(int j = 0; j < count; j++) {
        tmp = result.removeFirst();
        add(result, tmp, tr);
      }
    }

    return result;
  }

  public static void add(List<String> res, String str, Set<String> tr) {
    String tmp = "";
    for(int i = 0; i < str.length(); i++) {
      tmp = str.substring(0, i+1) + "()" + str.substring(i+1);
      if(!tr.contains(tmp)) {
        tr.add(tmp);
        res.add(tmp);
      }
    }
  }

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Course: https://www.educative.io/collection/5668639101419520/5671464854355968
Lesson: https://www.educative.io/collection/page/5668639101419520/5671464854355968/5753264117121024