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17. Reverse Linked List

easyAsked at Adyen

Given the head of a singly linked list, reverse the list 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 reversed list's head.

Constraints

  • 0 <= list length <= 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. Recursive

Recurse to tail, then rewire pointers on the way back.

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

Tradeoff:

2. Iterative pointer flip

Walk forward and reverse next pointers in place with three pointers.

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

Tradeoff:

Adyen-specific tips

Adyen expects the iterative O(1)-space variant first — they avoid recursion in payment-flow code so a stack-blowing 5000-node list is exactly the constraint they probe.

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Output

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