In Java, why is insertion or deletion in a Linked List a constant time operation? Isn't it misleading?

Question

Insertion or deletion of an element at a specific point of a list, assuming that we have a pointer to the node already, is a constant-time operation. - from

Answer 1

First of: the LinkedList implemented in the Sun JDK effectively has a link to the last element as well as to the first element (there's only a head entry, but head.previous points to the last element). This means that even in the worst case navigating through a list to an element indicated by an index should take n/2 operations. It's also a doubly linked list.

Apart from that: inserting into the beginning or end of a LinkedList is trivially O(1), because you don't need to traverse all elements.

Inserting/removing anywhere else depends on how exactly you do it! If you use an Iterator (of a ListIterator for adding) then the operation can be O(1) as well, as the Iterator will already have a reference to relevant entry.

If, however, you are using add(int, E) or remove(int), then the LinkedList will have to find the relevant entry (O(n)) and then remove the element (O(1)), so the entire operation will be O(n).