
# Reverse Linked List

## Description

[doc](https://leetcode.com/problems/reverse-linked-list/)

## Solutions

### First Idea

```javascript
/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var reverseList = function (head) {
  if (!head) {
    return head
  }

  let prev = null
  let cursor = head

  while (cursor.next) {
    let next = cursor.next
    cursor.next = prev

    prev = cursor
    cursor = next
  }

  cursor.next = prev
  return cursor
}
```

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

### Refined Version

```javascript
/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var reverseList = function (head) {
  let prev = null
  let cur = head

  while (cur) {
    let next = cur.next
    cur.next = prev

    prev = cur
    cur = next
  }

  return prev
}
```

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