Skip to main content

5. Reverse Linked List

easyAsked at Redis

Reverse a singly linked list in place; Redis uses it as a warm-up before diving into Redis list internals like the quicklist and listpack.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given the head of a singly linked list, reverse the list and return the new head. Solve both iteratively and (optionally) recursively.

Constraints

  • 0 <= nodes <= 5000
  • -5000 <= Node.val <= 5000

Examples

Example 1

Input
head = [1,2,3,4,5]
Output
[5,4,3,2,1]

Example 2

Input
head = []
Output
[]

Approaches

1. Stack rebuild

Push all nodes to a stack then pop.

Time
O(n)
Space
O(n)
const stack = [];
let cur = head;
while (cur) { stack.push(cur); cur = cur.next; }
const dummy = new ListNode();
let tail = dummy;
while (stack.length) { tail.next = stack.pop(); tail = tail.next; }
tail.next = null;
return dummy.next;

Tradeoff:

2. Iterative pointer flip

Walk the list keeping prev/cur/next and re-point each node. This is exactly how Redis lists used to work before quicklist replaced the raw linked list.

Time
O(n)
Space
O(1)
function reverseList(head) {
  let prev = null, cur = head;
  while (cur) {
    const next = cur.next;
    cur.next = prev;
    prev = cur;
    cur = next;
  }
  return prev;
}

Tradeoff:

Redis-specific tips

Redis interviewers love a quick aside on quicklist (linked list of listpacks) here; show you know Redis dropped raw doubly-linked lists for cache locality.

Solve it now

Free. No sign-up. Python and JavaScript run instantly in your browser.

Output

Press Run or Cmd+Enter to execute

Practice these live with InterviewChamp.AI

Drill Reverse Linked List and other Redis interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →