[数据结构与算法]21. 合并两个有序链表

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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 mergeTwoLists(self,l1:ListNode,l2:ListNode) -> ListNode:
        if l1 is None:
            return l2
        elif l2 is None:
            return l1

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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