C Linked-list Destroy Function

问题:

I am trying to learn C, and as many people, I have been a little stuck with pointers. Anyways, I made a recursive function that destroys my linked list, but as I have debugged, when I am back from the function the head of the list is not null as it should be, so I'm guessing it's some basic misunderstanding with pointers. Here's the function:

void destroy(struct node *n) {
   if (!n) return;
   destroy(n->next);
   free(n);
   n = NULL;
}

回答1:

void deleteList(struct node** head_ref)
{  
  struct node* current = *head_ref;
  struct node* next;
  while (current != NULL) {
    next = current->next;
    free(current);
    current = next;
  }
  *head_ref = NULL;
}

Try like this ....you can change names as you want. If you still need help let me know.

回答2:

Head has been freed when this functions ends but it's not null. Everything in C is passed by value. So you pass a copy of the location of head into destroy. That memory is deallocated but head is not changed.

You could write this as:

destroy(&head);

void destroy(struct node** n){
   if(!*n) return;
   destroy(&((*n)->next));
   free(*n);
   *n = NULL; 
}

回答3:

You have to use a pointer pointing to your list, calling with destroy(&n):

// clear complete list 
void destroy(struct node **n)
{
    if (*n == NULL)
        return;

    if ((*n)->next == NULL)
    {
        free(*n);
        *n= NULL;
        return;
    }

    struct node *iter = *n;
    struct node *prev = NULL;

    // get last item and the previous one
    while (iter->next != NULL)
    {
        prev = iter;
        iter = iter -> next;
    } 

    prev->next = NULL;
    free(iter);

    destroy(n);
}

Hope this may help you.

回答4:

Your recursive destroy function cannot modify the head variable in the caller's frame.

The statement n = NULL only affects the function argument, which is a local variable of the destroy function. It actually has no effect, so you can remove this statement.

You should set the head to NULL after the call to destroy in the caller function if it is needed there.

回答5:

Here is the sample function to destroy Linked list using DeleteRear():

void Destroy_Using_Rear(List *L)
{
    int y;
    Node *P,*Q,*Z;

    while(P!=NULL){
        y=DeleteRear(L,x);
        return y;
        Z=P;
        P=*L;
    }
}