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：
输入：1->2->4, 1->3->4
输出：1->1->2->3->4->4
题解：一个为None后即可结束，剩下的补充上
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, x):
# self.val = x
# self.next = None
class Solution:
def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
dummy = ListNode(-1)
head = dummy
while l1 and l2:
if l1.val <= l2.val:
head.next = ListNode(l1.val)
head = head.next
l1 = l1.next
else:
head.next = ListNode(l2.val)
head = head.next
l2 = l2.next
head.next = l1 if l1 else l2
return dummy.next
