206. Reverse Linked List

Solution

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;
  }
}

const first = new ListNode(1);
const second = new ListNode(2);
const third = new ListNode(3);
const fourth = new ListNode(4);
const fifth = new ListNode(5);

first.next = second;
second.next = third;
third.next = fourth;
fourth.next = fifth;

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

  while(current) {
    const next = current.next;
    current.next = prev;
    prev = current;
    current = next;
  }

  return prev;
}

console.log(reverseList(first));