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static List<string> FindUniqueElements(string[] arr)
    {
        HashSet<string> allElements = new HashSet<string>();
        HashSet<string> uniqueElements = new HashSet<string>();
        
        foreach (string word in arr)
        {
            if (allElements.Contains(word))
            {
                uniqueElements.Remove(word);
            }
            else
            {
                allElements.Add(word);
                uniqueElements.Add(word);
            }
        }

        return new List<string>(uniqueElements);