Insert at Head - Dart DSA

Insert a new first node by pointing it at the old head and then moving the head pointer.

Algorithm

Basic Implementation

basic.dart

class Node {
  int value;
  Node? next;
  Node(this.value, [this.next]);
}

String render(Node? head) {
  final parts = <String>[];
  var cursor = head;
  while (cursor != null) {
    parts.add(cursor.value.toString());
    cursor = cursor.next;
  }
  return '${parts.join(' -> ')} -> null';
}

void main() {
  var head = Node(20, Node(30));
  final newHead = Node(10);
  newHead.next = head;
  head = newHead;
  print(render(head));
}

Head insertion changes only two references: the new node points at the old head, then head moves to the new node.

Step 1 - Old first node

Before insertion, head points at node(20).

Step 2 - New node links to old head

Set new.next to the old first node before moving head.

Step 3 - Head moves to the new node

The final chain has 10 first: 10 -> 20 -> 30 -> null.

Complexity

Time: O(1)
Space: O(1)

Implementation notes

Keep the explicit node and pointer/reference operations; array shortcuts hide the linked-list state this lesson is meant to replay.
The final output prints the chain in a deterministic a -> b -> null form for cross-language comparison.

old head The previous first node becomes the second node.

constant-time insert Only the new node and head pointer change.