python leetcode21合并两个有序链表

将两个有序链表合并为一个新的有序链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。 

示例：

输入：1->2->4, 1->3->4
输出：1->1->2->3->4->4

python代码

# 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:#l1为空时直接返回l2
            return l2
        if l2 is None:
            return l1
        head=ListNode(0)#创建一个新链表，此为头节点
        current=head
        while l1 is not None and l2 is not None:
            if l1.val<l2.val:
                current.next=l1
                l1=l1.next
            else:
                current.next=l2
                l2=l2.next
            current=current.next
        if l1 is None:#当l1循环到为空时
            current.next=l2
        elif l2 is None:
            current.next=l1
        return head.next####