LeetCode: Reverse Linked List Solution

Approach

Update head until the initial head is tail

• Stay with the initial head
• Update new head as the initial head's next
• Each new updated head's next is the previous head

1Input: 1 (initial head) -> 2 -> 3 -> 4
2
3After iteration 1: 2 -> (1) -> 3 -> 4
4After iteration 2: 3 -> 2 -> (1) -> 4
5
6Output: 4 -> 3 -> 2 -> (1)

Implementation (iterative)

1var reverseList = function (head) {
2  const curr = head
3
4  while (curr && curr.next) {
5    let next = curr.next
6
7    curr.next = curr.next.next
8    next.next = head
9
10    head = next
11  }
12
13  return head
14}

Implementation (recursive)

1var reverseList = function (head) {
2  if (!head || !head.next) return head
3  const newHead = reverseList(head.next)
4  head.next.next = head
5  head.next = null
6  return newHead
7}