Reverse Linked List

Given the head of a singly linked list, reverse the list in-place and return the new head.

Input:  head = [1, 2, 3, 4, 5]
Output: [5, 4, 3, 2, 1]
Explanation: The links are reversed one by one.

export class ListNode {
  val: number;
  next: ListNode | null;

  constructor(val?: number, next?: ListNode | null) {
    this.val = val ?? 0;
    this.next = next ?? null;
  }
}

export function reverseList(head: ListNode | null): ListNode | null {
  let prev: ListNode | null = null;
  let current = head;

  while (current !== null) {
    const nextTemp = current.next; // store next node
    current.next = prev;           // reverse pointer
    prev = current;                // move prev forward
    current = nextTemp;            // move current forward
  }

  return prev;
}