Why is deleting in a single linked list O(1)?

Question

I do not quiet understand why deleting at the end of a single linked list goes in O(1) time, as the wikipedia article says.

A single linked list consists out of node

Answer 1

If you're including the cost of fixing the dangling node, you can still do it in O(1) with the use of sentinel node for the end (also described on that page).

Your "empty" list starts with a single sentinel

Head -> [Sentinel]

Add some stuff

Head -> 1 -> 2 -> 3 -> [Sentinel] 

Now delete the tail (3) by marking the node that was 3 as invalid, and then removing the link to the old sentinel, and freeing the memory for it:

Head -> 1 -> 2 -> 3 -> [Sentinel] 
Head -> 1 -> 2 -> [Sentinel] -> [Sentinel] 
Head -> 1 -> 2 -> [Sentinel]