What is the pointer-to-pointer technique for the simpler traversal of linked lists? [duplicate]

Answer 1

By "double-pointer", I think you mean "pointer-to-pointer". This is useful because it allows you to eliminate special cases for either the head or tail pointers. For example, given this list:

struct node {
    struct node *next;
    int key;
    /* ... */
};

struct node *head;

If you want to search for a node and remove it from the list, the single-pointer method would look like:

if (head->key == search_key)
{
    removed = head;
    head = head->next;
}
else
{
    struct node *cur;

    for (cur = head; cur->next != NULL; cur = cur->next)
    {
        if (cur->next->key == search_key)
        {
            removed = cur->next;
            cur->next = cur->next->next;
            break;
         }
    }
}

Whereas the pointer-to-pointer method is much simpler:

struct node **cur;

for (cur = &head; *cur != NULL; cur = &(*cur)->next)
{
    if ((*cur)->key == search_key)
    {
        removed = *cur;
        *cur = (*cur)->next;
        break;
    }
}