Why does node.val work without node! inside the loop?

Dart promotes a local variable after a null check: inside while (current != null) or a for with node != null, the variable has the non-nullable type ListNode. Promotion only applies to locals and private final fields, which is why the solutions copy head.next into a local before testing it.

Why can I not null-check head.next directly and then use it?

next is a public mutable field, so Dart cannot be sure it still holds the same value at the next read and does not promote it. final second = head?.next; reads it once into a local, and the local is promoted after second == null is ruled out.

What does head?.next return for an empty list?

The null-aware operator ?. short-circuits: if head is null, head?.next is null instead of throwing. That folds the empty-list check and the single-node check into one condition.

Is removeLast on a Dart List O(1)?

Yes. A growable List is backed by an array, and removing the last element only shrinks its length. removeAt(0) would shift every element and cost O(n) per call.

Does Dart have a built-in linked list?

dart:collection has LinkedList >, a doubly linked list of entry objects, and Queue. Neither is a ListNode chain; here the task is to rewire the given nodes, and for a plain List there is list.reversed, a lazy reversed view.

Reverse a Linked List in Dart: Iterative and Recursive Solutions

reverse_list.dart

Dart 3.7

Not run yet

+ 7 hidden

Input

head = [3, 6, 9, 12, 15]

Expected

[15, 12, 9, 6, 3]

The old tail 15 becomes the new head, and the old head 3 now ends the list.