Post a new vectorint containing the keys in the

* POST: a new vector<int> containing the keys in the correct order has been
returned.
*/
std::vector<int> to_vector(Node* head) {
std::vector<int> result;
for (Node* curr = head; curr != NULL; curr = curr->next){
result.push_back(curr->key);
}
return result;
}
/**
* Delete the last Node in the linked_list
* PRE: head is the first Node in a linked_list (if NULL, linked_list is empty)
* POST: the last Node of the linked_list has been removed
* POST: if the linked_list is now empty, head has been changed
* POST: else head remains the first Node in the linked_list
*/
void delete_last_element(Node*& head){
if (head != NULL) {
if (head -> next == NULL) {

head = NULL;
delete head;
} else {
delete_last_element(head->next);
}
}
}
/**
* Removes an existing Node (with key=oldKey) from the linked_list.
* PRE: head is the first node in a linked_list (if NULL, linked_list is empty)
* PRE: oldKey is the value of the key in the Node to be removed
* PRE: if no Node with key=oldKey exists, the linked_list has not changed
* POST: if a Node with key=oldKey was found, then it was deleted
* POST: other Nodes with key=oldKey might still be in the linked_list
* POST: head is the new first Node of the linked_list, if updated
*/
void remove(Node*& head, int oldKey) {
if (head != NULL) {
if (head->key == oldKey) {
Node* n = head->next;