Question

Is there any general approach if one wanted to provide an immutable version of e.g. LinkidList, implemented using as a linked sequence of nodes? I understand that in the case of ArrayList you would copy the underlying array, but in this case this is not that obvious to me…

Answer 1

Immutable lists are basically represented the same way as regular linked lists, except that all operations that would normally modify the list return a new one instead. This new list does not neccessarily need to contain a copy of the entire previous list but can reuse elements of it.

I recommend implementing the following operations in the following ways:

• Popping the element at the front: simply return a pointer to the next node. Complexity: O(1).
• Pushing an element to the front: Create a new node that point to the first node of the old list and return it. O(1).
• Concatenating list a with list b: copy the entire list a and let the pointer in the final node point to the beginning of list b. Note that this is faster than the same operation on mutable lists. O(length(a)).
• Inserting at position x: Copy everything up to x, add a node with the new element to the back of the copy, and let that node point to the old list at position x + 1. O(x).
• Removing the element at position x: practically the same as inserting. O(x).
• Sorting: you can just use plain quick- or mergesort. It’s not much faster or slower than it would be on mutable lists. The only difference is that you can’t sort in place but will have to sort to a copy. O(n*log n).