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

Is to merge two linked lists , Arrange them from small to large

The better way in time and space is ：

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

keep l1 It is a linked list connecting the smallest node , Through constant exchange l1 and l2 To achieve .