/*
 * @lc app=leetcode id=21 lang=javascript
 *
 * [21] Merge Two Sorted Lists
 *
 * https://leetcode.com/problems/merge-two-sorted-lists/description/
 *
 * algorithms
 * Easy (46.02%)
 * Total Accepted:    562.7K
 * Total Submissions: 1.2M
 * Testcase Example:  '[1,2,4]\n[1,3,4]'
 *
 * Merge two sorted linked lists and return it as a new list. The new list
 * should be made by splicing together the nodes of the first two lists.
 *
 * Example:
 *
 * Input: 1->2->4, 1->3->4
 * Output: 1->1->2->3->4->4
 *
 *
 */
/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var mergeTwoLists = function(l1, l2) {
  let current = new ListNode();
  const dummy = current;

  while (l1 || l2) {
    if (!l1) {
      current.next = l2;
      return dummy.next;
    } else if (!l2) {
      current.next = l1;
      return dummy.next;
    }

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

    current = current.next;
  }

  return dummy.next;

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