/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
export const mergeTwoLists = function (l1, l2) {
  // src/list/merge-k-sorted-lists.js 中已经写过了
  const dummyHead = {}
  let current = dummyHead
  while (l1 !== null && l2 !== null) {
    if (l1.val < l2.val) {
      if (l1.val !== null) {
        current.next = l1
        current = current.next
      }

      l1 = l1.next
    } else {
      if (l2.val !== null) {
        current.next = l2
        current = current.next
      }

      l2 = l2.next
    }
  }

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

  return dummyHead.next
}