Reverse a Singly Linked List

Walk the list with three references: 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 -> null reverses to 5 -> 4 -> 3 -> 2 -> 1 -> null after five rewire frames.

Basic Implementation

basic.js

class Node {
    constructor(value, next) {
        this.value = value;
        this.next = next === undefined ? null : next;
    }
}

const n5 = new Node(5, null);
const n4 = new Node(4, n5);
const n3 = new Node(3, n4);
const n2 = new Node(2, n3);
let head = new Node(1, n2);

let prev = null;
let cursor = head;
while (cursor !== null) {
    const next = cursor.next;
    cursor.next = prev;
    prev = cursor;
    cursor = next;
}
head = prev;

Complexity

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

Implementation notes

JavaScript: same three-pointer pattern as the other languages. 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.

three pointers `prev` starts `null`, `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`).