题目
You are given the heads of two sorted linked lists list1 and list2.
Merge the two lists into one sorted list. The list should be made by splicing together the nodes of the first two lists.
Return the head of the merged linked list.
解题思路
创建一个虚拟节点指向结果，两个指针，每次将较小的数，连接到结果
代码
class Solution:
def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
dummy = ListNode(0)
pre = dummy
while l1 and l2:
if l1.val < l2.val:
pre.next = l1
l1 = l1.next
else:
pre.next = l2
l2 = l2.next
pre = pre.next
pre.next = l1 if l1 is not None else l2
return dummy.next
