0021_merge_two_sorted_lists.js

/**
 * @number 21
 * @title Merge Two Sorted Lists
 * @url https://leetcode.com/problems/merge-two-sorted-lists/
 * @difficulty easy
 */

const mergeTwoLists = function (l1, l2) {
  let dummy = new ListNode(0);
  let current = dummy;

  while (l1 && l2) {
    if (l1.val < l2.val) {
      current.next = l1;
      l1 = l1.next;
    } else {
      current.next = l2;
      l2 = l2.next;
    }

    current = current.next;
  }

  if (l1) {
    current.next = l1;
  } else {
    current.next = l2;
  }

  return dummy.next;
};