unique method

Implementation

void unique() {
  if (_head == null) return;
  _ForwardListNode<T>? current = _head;
  while (current != null && current.next != null) {
    if (current.value == current.next!.value) {
      current.next = current.next!.next;
      _length--;
    } else {
      current = current.next;
    }
  }
}