题目链接

思路

参考官方题解的迭代法

var mergeTwoLists = function (list1, list2) {
  let dummy = new ListNode(0);
  let prev = dummy;
  while (list1 && list2) {
    if (list1.val <= list2.val) {
      prev.next = list1;
      list1 = list1.next;
    } else {
      prev.next = list2;
      list2 = list2.next;
    }
    prev = prev.next;
  }
  prev.next = list1 === null ? list2 : list1;
  return dummy.next;
};