Linked Structures

Reverse a Singly Linked List

Walk the list with three pointers: prev, cursor, and next. Each iteration saves cursor.next, re-points cursor.next backward to prev, then advances prev = cursor; cursor = next.

Algorithm

Basic Implementation

basic.scala 
import scala.collection.mutable.ArrayBuffer

class ListNode(var value: Int, var next: Int)

object Main {
	def addNode(nodes: ArrayBuffer[ListNode], value: Int, next: Int): Int = {
		val idx = nodes.length
		nodes += new ListNode(value, next)
		idx
	}

	def main(args: Array[String]): Unit = {
		val nodes = ArrayBuffer.empty[ListNode]
		val n5 = addNode(nodes, 5, -1)
		val n4 = addNode(nodes, 4, n5)
		val n3 = addNode(nodes, 3, n4)
		val n2 = addNode(nodes, 2, n3)
		var head = addNode(nodes, 1, n2)

		var prev = -1
		var cursor = head
		while (cursor != -1) {
			val next = nodes(cursor).next
			nodes(cursor).next = prev
			prev = cursor
			cursor = next
		}
		head = prev
		var cur = head
		while (cur != -1) {
			print(s"${nodes(cur).value} -> ")
			cur = nodes(cur).next
		}
		println("nil")
	}
}

The three-pointer loop saves the forward link, flips one next pointer, then advances prev and cursor.

Step 1 - Save the first forward link

prev starts at null, cursor is node(1), and nxt saves node(2).

Initial 1 -> 2 -> 3 -> 4 -> 5 chain with prev, cursor, and nxt named.prevcursornxtnullnode(1)node(2)node(3)node(4)node(5)

Step 2 - Flip node(1)

Set node(1).next to prev, making the reversed prefix 1 -> null.

After the first flip, prev points at node(1) and cursor advances to node(2).prevcursornode(1)nullnode(2)node(3)node(4)node(5)

Step 3 - Reversed prefix reaches 3

After three flips, the prefix is 3 -> 2 -> 1 -> null and cursor is node(4).

Middle of the reverse: prefix 3 -> 2 -> 1, suffix 4 -> 5.prevcursornode(3)node(2)node(1)nullnode(4)node(5)

Step 4 - Done

When cursor reaches null, prev is the new head: 5 -> 4 -> 3 -> 2 -> 1 -> null.

Final reversed list.headnode(5)node(4)node(3)node(2)node(1)null

Complexity

  • Time: O(n)
  • Space: O(1)

Implementation notes

  • Scala: same three-pointer pattern as the other languages, but each pointer is an Int index into the ArrayBuffer[ListNode] arena (-1 is the sentinel). The arena avoids the Option[ListNode] ownership dance the classic reference idiom requires while keeping the algorithm visible.
  • head = prev at the end re-points the head at the old tail; the arena keeps every node alive throughout the reversal.
  • The replay shows all three pointers each frame and a distinct rewire frame between save and advance, with node(<value>) labels instead of runtime references.
three pointers `prev` starts `-1`, `cursor` starts at `head`, `next` is the saved forward link.
rewire The rewire frame flips `cursor.next` from forward (toward `next`) to backward (toward `prev`).