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28. Reverse Linked List

easyAsked at Plaid

Reverse a singly-linked list. Plaid asks this as a pointer-juggling baseline before harder list problems on transaction history sequences.

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

Source citations

Public interview reports confirming this problem appears in Plaid loops.

  • Glassdoor (2025)— Plaid SWE I onsite warm-up.
  • Blind (2026)— Plaid backend OA — list reversal.

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. Collect values, rebuild reversed

Walk the list into an array, reverse, rebuild a new list.

Time
O(n)
Space
O(n)
function reverseList(head) {
  const vals = [];
  for (let n = head; n; n = n.next) vals.push(n.val);
  let dummy = { next: null }, cur = dummy;
  for (let i = vals.length - 1; i >= 0; i--) {
    cur.next = { val: vals[i], next: null };
    cur = cur.next;
  }
  return dummy.next;
}

Tradeoff: Allocates a new list. Plaid wants the in-place pointer reversal.

2. Three-pointer in-place reversal

Walk the list. At each node, flip its next to point to the previous node. Track prev, curr, next.

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

Tradeoff: Linear time, in-place. The save-next-before-flipping is the key — without it you lose the rest of the list.

Plaid-specific tips

Plaid grades this on the pointer dance — the order of the four assignments matters. Articulate 'save next first, then flip curr.next, then advance' before coding. Bonus signal: ship the recursive version (head.next.next = head, head.next = null) as an alternative to demonstrate fluency, but mention it costs O(n) stack space.

Common mistakes

  • Flipping curr.next before saving the original next — loses the rest of the list.
  • Returning head instead of prev — head is now the tail with .next === null.
  • Forgetting to handle empty input — null prev returned is correct.

Follow-up questions

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

  • Reverse a sublist between positions m and n (LC 92).
  • Reverse in groups of k (LC 25).
  • Reverse a doubly-linked list.

Solve it now

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Output

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FAQ

Why save next before flipping?

Setting curr.next = prev overwrites the original next pointer. Without saving it, you can't advance to the next node.

Recursive vs iterative — which does Plaid prefer?

Iterative for production (no stack-overflow risk on 1M-node histories), recursive for the elegant explanation. Lead with iterative.

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