将两个升序链表合并为一个新的 升序 链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。

示例 1：

输入：l1 = [1,2,4], l2 = [1,3,4]
输出：[1,1,2,3,4,4]

示例 2：

输入：l1 = [], l2 = []
输出：[]

示例 3：

输入：l1 = [], l2 = [0]
输出：[0]

解法

双指针法

感觉有点暴力，没意思。

class Solution:
    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
        if list1 is None:
            return list2
        if list2 is None:
            return list1
        a = ListNode(0)
        result = a
        while list1 is not None or list2 is not None:
            if list1 is None and list2:
                result.next = list2
                list2 = list2.next
            elif list2 is None and list1:
                result.next = list1
                list1 = list1.next
            elif list1.val < list2.val:
                result.next = list1
                list1 = list1.next
            else:
                result.next = list2
                list2 = list2.next
            result = result.next
        return a.next

递归法

class Solution:
    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
        if list1 is None:
            return list2
        elif list2 is None:
            return list1
        if list1.val < list2.val:
            list1.next = self.mergeTwoLists(list1.next, list2)
            return list1
        list2.next = self.mergeTwoLists(list1,list2.next)
        return list2