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解法一（60 ms 89.73%）：

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        if l1 is None and l2 is None:
            return None
        new_list=ListNode(0)
        pre=new_list
        while l1 is not None and l2 is not None:
            if l1.val<l2.val:      
                pre.next=l1
                l1=l1.next
            else:
                pre.next=l2
                l2=l2.next  
            pre=pre.next
        if l1 is not None:
            pre.next=l1
        if l2 is not None:
            pre.next=l2
        return new_list.next