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.go 
package main

import "fmt"

type ListNode struct {
	Value int
	Next  *ListNode
}

func makeNode(v int, next *ListNode) *ListNode {
	return &ListNode{Value: v, Next: next}
}

func main() {
	n5 := makeNode(5, nil)
	n4 := makeNode(4, n5)
	n3 := makeNode(3, n4)
	n2 := makeNode(2, n3)
	head := makeNode(1, n2)

	var prev *ListNode = nil
	cursor := head
	for cursor != nil {
		next := cursor.Next
		cursor.Next = prev
		prev = cursor
		cursor = next
	}
	head = prev
	for cur := head; cur != nil; cur = cur.Next {
		fmt.Printf("%d -> ", cur.Value)
	}
	fmt.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

  • Go: same three-pointer pattern as the other languages. Each pointer is a *ListNode, and nil represents end-of-list honestly.
  • Reverse in place and reassign head = prev at the end.
  • The replay shows all three pointers each frame and a distinct rewire frame between save and advance, with node(<value>) labels instead of raw addresses.
three pointers `prev` starts `nil`, `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`).