328. Odd Even 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;
  }
}

function oddEvenList(head: ListNode | null): ListNode | null {
  if (!head?.next) {
    return head;
  }

  let odd = head;
  let even = head.next;

  const evenHead = even;

  while (even?.next) {
    odd.next = even.next;
    odd = odd.next;

    even.next = odd.next;
    even = even.next!;
  }

  odd.next = evenHead;

  return head;
}

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;

oddEvenList(first);