206. Reverse Linked List

At a Glance

• Topic: Linked List
• Pattern: In-place Reversal
• Difficulty: Easy
• LeetCode: 206

Problem Statement

Given the head of a singly linked list, reverse the list, and return the reversed list.

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: []

Constraints:

The number of nodes in the list is the range [0, 5000].
-5000 <= Node.val <= 5000

Follow up: A linked list can be reversed either iteratively or recursively. Could you implement both?

Approach & Solution Steps

Iterate through the list, maintaining prev, curr, and next pointers. At each step, update curr.next to point to prev, then advance prev and curr.

Optimal JS Solution

function reverseList(head) {
  let prev = null, curr = head;
  while (curr) {
    const next = curr.next;
    curr.next = prev;
    prev = curr;
    curr = next;
  }
  return prev;
}

Edge Cases & Pitfalls

• Always consider empty or null inputs.
• Watch out for off-by-one index errors.