Pre head is the first node in a linkedlist if null

* PRE: head is the first node in a linked_list (if NULL, linked_list is empty)
* 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){
// ******** WRITE YOUR CODE HERE ********
Node* curr = head;

Node* temp = head; // Stores the previous Node
if (curr == NULL)
return;
if (curr->next == NULL) {
delete curr;
curr = NULL;
return;
}
while (curr->next != NULL) {
temp = curr;
curr = curr->next;
}
delete curr; // Delete the last element
temp->next = NULL; // set the previous one's next to NULL
}
/**
* 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