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5. Reverse Linked List

easyAsked at Postman

Reverse a singly linked list in place and return the new head.

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

Problem

Given the head of a singly linked list, reverse the list and return the new head. Both iterative and recursive solutions are expected to be discussed.

Constraints

  • 0 <= nodes <= 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

Push all values onto a stack, then rebuild a new list in popped order.

Time
O(n)
Space
O(n)
const stack = [];
while (head) { stack.push(head.val); head = head.next; }
// build new list from stack pops

Tradeoff:

2. Iterative pointer flip

Walk the list, flipping each node's next pointer to its predecessor. Three pointers suffice.

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

Tradeoff:

Postman-specific tips

Postman engineers reverse linked lists when undoing request-history breadcrumbs in the app, so they expect both iterative and recursive forms verbally.

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Output

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