Delete a node in singly link list

Question

How to delete a node in a singly link list with only one pointer pointing to node to be deleted?

[Start and end pointers are not known, the available information is pointer to node which should be deleted]

Answer 1

You can delete a node without getting the previous node, by having it mimic the following node and deleting that one instead:

void delete(Node *n) {
  if (!is_sentinel(n->next)) {
    n->content = n->next->content;
    Node *next = n->next;
    n->next = n->next->next;
    free(next);
  } else {
    n->content = NULL;
    free(n->next);
    n->next = NULL;
  }
}

As you can see, you will need to deal specially for the last element. I'm using a special node as a sentinel node to mark the ending which has content and next be NULL.

UPDATE: the lines Node *next = n->next; n->next = n->next->next basically shuffles the node content, and frees the node: Image that you get a reference to node B to be deleted in:

   A           / To be deleted
  next   --->  B
              next  --->    C
                           next ---> *sentinel*

The first step is n->content = n->next->content: copy the content of the following node to the node to be "deleted":

   A           / To be deleted
  next   --->  C
              next  --->    C
                           next ---> *sentinel*

Then, modify the next points:

   A           / To be deleted
  next   --->  C       /----------------
              next  ---|    C          |
                           next ---> *sentinel*

The actually free the following element, getting to the final case:

   A           / To be deleted
  next   --->  C
              next  --->    *sentinel*

Answer 2

Not possible.

There are hacks to mimic the deletion.

But none of then will actually delete the node the pointer is pointing to.

The popular solution of deleting the following node and copying its contents to the actual node to be deleted has side-effects if you have external pointers pointing to nodes in the list, in which case an external pointer pointing to the following node will become dangling.

You can find some discussion on SO here.