69. Reverse Linked List II

mediumAsked at Ola

Reverse a linked list between positions left and right.

By Alex Chen, Founder, InterviewChamp.AI · Last verified 2026-05-20

Problem

Given the head of a singly linked list and two integers left and right where left <= right, reverse the nodes of the list from position left to position right and return the reversed list.

Constraints

Number of nodes is n
1 <= n <= 500
1 <= left <= right <= n

Examples

Example 1

Input

head = [1,2,3,4,5], left = 2, right = 4

Output

[1,4,3,2,5]

Example 2

Input

head = [5], left = 1, right = 1

Output

[5]

Approaches

1. Collect into array

Read values, reverse a slice, write back.

Time: O(n)
Space: O(n)

const arr=[]; let n=head; while(n){arr.push(n.val); n=n.next;}
let l=left-1, r=right-1; while(l<r){[arr[l],arr[r]]=[arr[r],arr[l]]; l++; r--;}
// write back arr to head node values

Tradeoff:

2. Iterative head-insertion

Move to position left-1, then iteratively pop the next node and insert it after prev. O(n) one pass.

Time: O(n)
Space: O(1)

function reverseBetween(head, left, right) {
  const dummy = { next: head };
  let prev = dummy;
  for (let i = 1; i < left; i++) prev = prev.next;
  const start = prev.next;
  for (let i = 0; i < right - left; i++) {
    const mv = start.next;
    start.next = mv.next;
    mv.next = prev.next;
    prev.next = mv;
  }
  return dummy.next;
}

Tradeoff:

Ola-specific tips

Ola uses this to test surgical pointer flips; tie it to reordering a contiguous slice of dispatch hops without affecting surrounding chain.

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Output

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