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.
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。
Input: list1 = [1,2,4], list2 = [1,3,4]
Output: [1,1,2,3,4,4]
思路：递归
class Solution:def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:if not l1: return l2 # 终止条件，直到两个链表都空if not l2: return l1if l1.val <= l2.val: # 递归调用l1.next = self.mergeTwoLists(l1.next,l2)return l1else:l2.next = self.mergeTwoLists(l1,l2.next)return l2
