DeleteList()

Q:

Write a function DeleteList() that takes a list, deallocates all of its memory and sets its
head pointer to NULL (the empty list).

A:

Delete the whole list and set the head pointer to NULL. There is a slight complication
inside the loop, since we need extract the .next pointer before we delete the node, since
after the delete it will be technically unavailable.
void DeleteList(struct node** headRef) {
struct node* current = *headRef; // deref headRef to get the real head
struct node* next;
while (current != NULL) {
next = current->next; // note the next pointer
free(current); // delete the node
current = next; // advance to the next node
}
*headRef = NULL; // Again, deref headRef to affect the real head back
// in the caller.
}