[数据结构与算法]21. 合并两个有序链表

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。?

输入：l1 = [1,2,4], l2 = [1,3,4]
输出：[1,1,2,3,4,4]

?

示例 1：

class Solution:
    def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
        if l1 is None:
            return l2
        elif l2 is None:
            return l1
        elif l1.val < l2.val:
            l1.next = self.mergeTwoLists(l1.next, l2)
            return l1
        else:
            l2.next = self.mergeTwoLists(l1, l2.next)
            return l2

?

class Solution:
    def mergeTwoSorted(self, l1: ListNode,l2:ListNode) ->ListNote:
        if l1 is None:
            return l2

?

class Solution:
    def mergeTwoLists(self,l1:ListNode,l2:ListNode) -> ListNode:
        if l1 is None:
            return l2
        elif l2 is None:
            return l1
        elif l1.val < l2.val:
            l1.next = self.mergeTwoLists(l1.next, l2)
            return l1
        else:
            l2.next = self.mergeTwoLists(l1,l2.next)
            return l2