Overview

This problem challenges you to reverse the order of nodes in a singly linked list. Given the head of a linked list, your task is to return the head of the reversed list. This fundamental operation is a common interview question and tests your understanding of pointer manipulation and linked list traversal.

What you need to do / Task

Implement a function reverseList that takes the head of a singly linked list as input and returns the new head of the list after its nodes have been reversed.

Input & output

Input:

• head: A ListNode object, representing the head of a singly linked list. It can be null if the list is empty.

Output:

• A ListNode object, representing the head of the reversed linked list. This will be null if the input list was empty.

ListNode Definition

For your convenience, the ListNode class is defined as follows:

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

(The JavaScript definition is similar.)

Clarifications

• Q: What should be returned if the input list is empty or contains only one node?
  • A: If the list is empty (head is null) or contains a single node, return the head itself. The reversal operation doesn't change these specific cases.
• Q: Should I create a new linked list or modify the existing one?
  • A: You must modify the existing linked list in-place. Do not create new ListNode objects for the reversal.
• Q: What about cycles in the linked list?
  • A: Assume there are no cycles in the input linked list.