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29. Reverse Linked List

easyAsked at Vercel

Reverse a singly linked list in-place. Vercel asks this constantly because it's the building block of every list-manipulation problem they care about — and because they want to see if you can do both iterative and recursive cleanly.

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

Source citations

Public interview reports confirming this problem appears in Vercel loops.

  • Glassdoor (2025-Q4)— Vercel screen warm-up; both iterative and recursive expected.
  • Blind (2026-Q1)— Listed in Vercel platform onsite recap.

Problem

Given the head of a singly linked list, reverse the list, and return the reversed list.

Constraints

  • The number of nodes in the list is the range [0, 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. Convert to array, reverse, rebuild

Walk to collect values; reverse the array; build a new list.

Time
O(n)
Space
O(n)
function reverseList(head) {
  const vals = [];
  let cur = head;
  while (cur) { vals.push(cur.val); cur = cur.next; }
  vals.reverse();
  let dummy = { val: 0, next: null }, tail = dummy;
  for (const v of vals) { tail.next = { val: v, next: null }; tail = tail.next; }
  return dummy.next;
}

Tradeoff: O(n) extra space and allocates n new nodes. Mention as the obvious non-answer.

2. Iterative three-pointer (optimal)

Walk the list; for each node, save next, point current at prev, slide prev/current forward.

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

Tradeoff: O(1) extra space, in-place. The three-pointer dance is the canonical answer for any list-reverse follow-up.

Vercel-specific tips

Vercel will accept the iterative version, then ask for recursion. Both should be ready: `if (!head || !head.next) return head; const rest = reverseList(head.next); head.next.next = head; head.next = null; return rest;`. Bonus signal: drawing the pointer diagram out loud as you write.

Common mistakes

  • Forgetting to save next BEFORE overwriting cur.next — loses the rest of the list.
  • Returning head instead of prev — returns the old head, which is now the tail.
  • In the recursive version, forgetting `head.next = null` — creates a 2-cycle at the original head.

Follow-up questions

An interviewer at Vercel may pivot to one of these next:

  • Reverse a range [m, n] (LC 92).
  • Reverse nodes in k-groups (LC 25, hard).
  • Reverse a doubly linked list.

Solve it now

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Output

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FAQ

Why three pointers?

You need prev (to attach to), cur (to modify), and next (to advance to). Without next, overwriting cur.next loses your forward pointer.

Iterative vs recursive — which does Vercel prefer?

Iterative for safety (O(1) stack). Be ready to write both — they often ask 'now do it recursively' after you finish the iterative.

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