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29. Reverse Linked List

easyAsked at Salesforce

Reverse a singly linked list. Salesforce uses this as a fundamental pointer-manipulation check — they expect both the iterative and recursive solutions.

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

Source citations

Public interview reports confirming this problem appears in Salesforce loops.

  • Glassdoor (2026-Q1)— Salesforce uses this as the canonical linked-list warmup before harder problems.
  • Blind (2025-12)— Recurring on Salesforce backend phone screens.

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

Example 3

Input
head = []
Output
[]

Approaches

1. Use a stack

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

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

Tradeoff: O(n) space and creates new nodes. Salesforce wants in-place pointer reversal.

2. Iterative pointer reversal

Walk the list with prev/curr/next pointers. At each step, flip curr.next to point to prev.

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: True in-place reversal. The next-pointer save is crucial — without it, you'd lose the rest of the list when you flip curr.next.

Salesforce-specific tips

Salesforce expects this in under 2 minutes — it's the floor for backend candidates. Bonus signal: also provide the recursive version (using the elegant 'reverse rest, then attach head' pattern) and discuss when each is preferred. They use linked-list reversal in their cursor-based pagination logic.

Common mistakes

  • Forgetting to save curr.next before reassigning — corrupts the list mid-walk.
  • Returning head instead of prev — head is now the last node, prev is the new head.
  • Initializing prev to head — flips the wrong direction.

Follow-up questions

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

  • Reverse Linked List II (LC 92) — reverse a sublist.
  • Reverse Nodes in k-Group (LC 25).
  • Palindrome Linked List (LC 234) — uses reversal as a subroutine.

Solve it now

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Output

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FAQ

Why save curr.next before reassigning curr.next?

Once you set curr.next = prev, the original curr.next is lost. Saving it lets you advance to the next node after flipping the pointer.

Iterative vs recursive — which does Salesforce prefer?

Iterative for production (no stack-overflow risk). They expect you to know both.

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