/**
 * Definition for singly-linked list.
 * 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)
 *     }
 * }
 */

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 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;
  }
  if (list1 !== null) {
    tail.next = list1;
  }
  if (list2 !== null) {
    tail.next = list2;
  }
  return dummy.next;
}
