Python, LeetCode, 21. 合并两个有序链表

# 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 == None:
            if l2 == None:
                return None
            else:
                return l2
        elif l2 == None:
            return l1
            
        prehead = ListNode(None)
        tmp = prehead
        while l1 != None and l2 != None:
            if l1.val < l2.val:
                tmp.next = l1
                tmp = l1
                l1 = l1.next
            else:
                tmp.next = l2
                tmp = l2
                l2 = l2.next
        if l1 == None:
            tmp.next = l2
        elif l2 == None:
            tmp.next = l1
            
        return prehead.next