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29. Reverse Linked List

easyAsked at Ola

Reverse a singly linked 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. Try both iterative and recursive.

Constraints

  • Number of nodes is in [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 collect

Push every node onto a stack, then pop and re-link.

Time
O(n)
Space
O(n)
const s = []; let n = head;
while (n) { s.push(n); n = n.next; }
const dummy = { next: null }; let t = dummy;
while (s.length) { t.next = s.pop(); t = t.next; }
if (t) t.next = null;
return dummy.next;

Tradeoff:

2. Iterative pointer flip

Walk with prev/curr/next pointers; flip curr.next each step. O(n) time and O(1) space.

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:

Ola-specific tips

Ola uses this to gauge pointer fluency; tie it to reversing a chain of dispatch hops when a trip is canceled mid-route.

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Output

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