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10. Reverse Linked List

easyAsked at Square

Reverse a singly linked list. Square uses this to test that you can manipulate next-pointers without losing nodes — the same discipline they expect when replaying out-of-order event logs during offline-to-online POS sync.

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

Source citations

Public interview reports confirming this problem appears in Square loops.

  • Glassdoor (2025)— Square Reader hardware-software interface team.
  • LeetCode Discuss (2026-Q1)— Cash App backend onsite warm-up.

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 in 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 = [1,2]
Output
[2,1]

Example 3

Input
head = []
Output
[]

Approaches

1. Recursion

Reverse the tail, then point head.next.next = head and head.next = null.

Time
O(n)
Space
O(n)
function reverseList(head) {
  if (!head || !head.next) return head;
  const newHead = reverseList(head.next);
  head.next.next = head;
  head.next = null;
  return newHead;
}

Tradeoff: Elegant but O(n) call stack — stack overflow on long lists, which Square interviewers do test.

2. Iterative three-pointer

Walk forward; at each node, redirect its next to the prev pointer.

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: Constant space. The 'save next before overwriting' move is the canonical pattern Square interviewers want to see.

Square-specific tips

Square interviewers grade this on whether you talk through pointer state with names ('prev', 'cur', 'next') and explain why you must save next BEFORE overwriting cur.next. Mention that this is the building block for reverse-in-groups (LC 25) which they sometimes follow up with. Drawing it on the whiteboard with arrows earns bonus signal.

Common mistakes

  • Overwriting cur.next before saving it — you lose the rest of the list.
  • Returning head at the end instead of prev — head now points to null.
  • Forgetting that the final cur is null (sentinel that lets prev be the new head).

Follow-up questions

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

  • Reverse nodes in K-group (LC 25).
  • Reverse a sub-range [m, n] (LC 92).
  • Reverse a doubly linked list.

Solve it now

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Output

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FAQ

Why save next before overwriting?

Once you do cur.next = prev, the original next pointer is gone. Saving it first lets you advance after the rewire.

Can I use a stack?

Yes — push all values, pop into a new list. But it's O(n) space without any code-clarity gain.

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