LinkedListInsertion.c

// Implementation without a guard:
int main() {
  struct Node* root = newNode(0);
  struct Node* newBeginning = newNode(-1);
  
  newBeginning -> next = root;
  root = newBeginning;
}

// Implementation using a guard:
int main() {
  struct Node* root = newNode(NULL);
  struct Node* firstRealElement = newNode(0);
  root -> next = firstRealElement;

  struct Node* newBeginning = newNode(-1);
  newBeginning->next = firstRealElement;
  root->next = newBeginning;
  // WE DON'T HAVE TO REASSIGN ROOT
}

// Implementation using Double Pointer
void insertAtBeginning(struct Node** head, int val) {
  struct Node* new = newNode(val);
  new->next = *head;
  *head = new;
}

int main() {
  struct Node* head = NULL; // empty list
  insertAtBeginning(&head, 10); // head = 10 -> NULL
  insertAtBeginning(&head, 20); // head = 20 -> 10 -> NULL
}