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.cs 
using System;
using System.Collections.Generic;

class ListNode {
	public int Value;
	public int Next;
}

class Program {
	static int AddNode(List<ListNode> nodes, int value, int next) {
		int idx = nodes.Count;
		nodes.Add(new ListNode { Value = value, Next = next });
		return idx;
	}

	static void Main() {
		List<ListNode> nodes = new List<ListNode>();
		int n5 = AddNode(nodes, 5, -1);
		int n4 = AddNode(nodes, 4, n5);
		int n3 = AddNode(nodes, 3, n4);
		int n2 = AddNode(nodes, 2, n3);
		int head = AddNode(nodes, 1, n2);

		int prev = -1;
		int cursor = head;
		while (cursor != -1) {
			int next = nodes[cursor].Next;
			nodes[cursor].Next = prev;
			prev = cursor;
			cursor = next;
		}
		head = prev;
		int cur = head;
		while (cur != -1) {
			Console.Write(nodes[cur].Value + " -> ");
			cur = nodes[cur].Next;
		}
		Console.WriteLine("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

  • C#: same three-pointer pattern as the other languages, but each pointer is an int index into the List<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`).