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:
            m=self.mergeTwoLists(l1.next,l2)
            l1.next=m
            return l1
        else:
            m=self.mergeTwoLists(l2.next,l1)
            l2.next=m
            return l2