singly linked list insert

void SinglyLinkedList::addLast(SinglyLinkedListNode *newNode) {
      if (newNode == NULL)
            return;
      else {
            newNode->next = NULL;
            if (head == NULL) {
                  head = newNode;
                  tail = newNode;
            } else {
                  tail->next = newNode;
                  tail = newNode;
            }
      }
}

void SinglyLinkedList::addFirst(SinglyLinkedListNode *newNode) {
      if (newNode == NULL)
            return;
      else {
            if (head == NULL) {
                  newNode->next = NULL;
                  head = newNode;
                  tail = newNode;
            } else {
                  newNode->next = head;
                  head = newNode;
            }
      }
}

void SinglyLinkedList::insertAfter(SinglyLinkedListNode *previous, SinglyLinkedListNode *newNode) {
      if (newNode == NULL)
            return;
      else {
            if (previous == NULL)
                  addFirst(newNode);
            else if (previous == tail)
                  addLast(newNode);
            else {
                  SinglyLinkedListNode *next = previous->next;
                  previous->next = newNode;
                  newNode->next = next;
            }
      }
}