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8. Reverse Linked List

easyAsked at Snap

Reverse a singly-linked list in place. Snap uses this to verify pointer-juggling fluency and ability to articulate both iterative and recursive solutions.

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

Source citations

Public interview reports confirming this problem appears in Snap loops.

  • Glassdoor (2026-Q1)— Snap reports include this as a 10-minute screen problem.
  • Blind (2025-11)— Mentioned alongside cycle-detection problems at Snap.

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. Stack-based

Push all nodes onto a stack, then pop them into a new list.

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

Tradeoff: Linear extra space. Works but the iterative pointer flip is strictly better.

2. Three-pointer iterative flip (optimal)

Maintain prev, curr, next. For each node, save next, flip curr.next to prev, advance prev and curr.

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

Tradeoff: O(1) extra space. The 'save next first' step is the crux — without it you lose the rest of the list.

Snap-specific tips

At Snap, candidates who can sketch both iterative AND recursive versions earn higher signal — recursion shows you understand the stack model. Bonus: relate this to reversing a Snap conversation thread for replay, where reversing in-place avoids re-renders.

Common mistakes

  • Forgetting to save next before flipping — loses the tail.
  • Returning curr (which is null) instead of prev (the new head).
  • Mutating during traversal without a temp variable for next.

Follow-up questions

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

  • Reverse a sublist between positions m and n (LC 92).
  • Reverse nodes in k-groups (LC 25).
  • Detect if a list is a palindrome using reversal of the second half.

Solve it now

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Output

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FAQ

Is recursion cleaner or more error-prone here?

Recursive code is shorter but uses O(n) stack. For 5000-node inputs it's safe; for arbitrary depths the iterative version is safer in production.

Does this work on doubly-linked lists?

Almost — you also need to flip the prev pointer at each step. The skeleton is the same.

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