Merge Two Sorted Lists

You are given the heads of two sorted linked lists list1 and list2.

Merge the two lists in a one sorted list. The list should be made by splicing together the nodes of the first two lists.

Return the head of the merged linked list.

Input: list1 = [1,2,4], list2 = [1,3,4]
Output: [1,1,2,3,4,4]

const mergeTwoLists = function (list1, list2) {
  if (!list1) return list2;
  if (!list2) return list1;

  const res = new ListNode();
  let cur = res;

  while (list1 && list2) {
    const v1 = list1.val;
    const v2 = list2.val;

    if (v1 <= v2) {
      cur.next = list1;
      list1 = list1.next;
    } else if (v1 > v2) {
      cur.next = list2;
      list2 = list2.next;
    }

    cur = cur.next;
  }

  cur.next = list1 || list2;

  return res.next;
};

// Recursive
const mergeTwoLists = function (l1, l2) {
  if (!l1) return l2;
  if (!l2) return l1;

  if (l1.val > l2.val) {
    l2.next = mergeTwoLists(l1, l2.next);
    return l2;
  } else {
    l1.next = mergeTwoLists(l1.next, l2);
    return l1;
  }
};