Merge Two Sorted Lists

Given the heads of two sorted linked lists, merge them into one sorted list and return its head. The merge must be done in-place, by re-linking the existing nodes.

Input:  list1 = [1, 2, 4], list2 = [1, 3, 4]
Output: [1, 1, 2, 3, 4, 4]
Explanation: Nodes are merged in sorted order.

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

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

export function mergeTwoLists( list1: ListNode | null, list2: ListNode | null): ListNode | null {
  const dummy = new ListNode();
  let tail = dummy;

  while (list1 !== null && list2 !== null) {
    if (list1.val < list2.val) {
      tail.next = list1;
      list1 = list1.next;
    } else {
      tail.next = list2;
      list2 = list2.next;
    }
    tail = tail.next;
  }

  tail.next = list1 !== null ? list1 : list2;

  return dummy.next;
}