public String toString () // Time complexity: O(n), Space complexity: O(1)
    {

        String str = "{";
        IntNode current = _head;
        while (current != null) // Goes through the set and print each element.
        {
            str += current.getValue() + ",";
            current = current.getNext();
        }
        str += + "}"; // Closes the set with closing bracket.
        return str;
    }