Insert at Head - Scala DSA

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

Algorithm

The replay labels nodes by value, such as node(20), and never exposes object identity or memory addresses. This Scala DSA implementation uses the same small chain as the rest of the DSA track.

old head The previous first node becomes the second node.

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

Visual walkthrough

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.

Basic Implementation

                            basic.scala
                        

                            class Node(val value: Int, var next: Node = null)

def render(head: Node): String = {
  val parts = scala.collection.mutable.ArrayBuffer[String]()
  var cursor = head
  while (cursor != null) {
    parts += cursor.value.toString
    cursor = cursor.next
  }
  parts.mkString(" -> ") + " -> null"
}

var head = new Node(20, new Node(30))
val newHead = new Node(10)
newHead.next = head
head = newHead
println(render(head))

                        

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.