21. Merge Two Sorted Lists

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

就是合并两个链表，让他们从小到大排列

时间空间上比较优秀的做法是：

def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
     if l1 and l2:
         if l1.val>l2.val:
             l1,l2=l2,l1
         l1.next=self.mergeTwoLists(l1.next,l2)
     return l1 or l2

保持l1是连接最小结点的一个链表，通过不断交换l1和l2来实现。