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

Canonical input 1 -> 2 -> 3 -> 4 -> 5 -> nil reverses to 5 -> 4 -> 3 -> 2 -> 1 -> nil after five rewire frames.

Basic Implementation

basic.kt

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

fun addNode(nodes: MutableList<ListNode>, value: Int, next: Int): Int {
	val idx = nodes.size
	nodes.add(ListNode(value, next))
	return idx
}

fun main() {
	val nodes = mutableListOf<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("${nodes[cur].value} -> ")
		cur = nodes[cur].next
	}
	println("nil")
}

Complexity

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

Implementation notes

Kotlin: same three-pointer pattern as the other languages, but each pointer is an Int index into the MutableList<ListNode> arena (-1 is the sentinel). The arena avoids the 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`).