LeetCode 21. 合并两个有序链表(Python)

题目：

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

解答：

方法一：

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

class Solution:
    def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
        head = ListNode(0)
        first = head
        while l1 and l2:
            if l1.val > l2.val:
                head.next = l2
                l2 = l2.next
            else :
                head.next = l1
                l1 = l1.next
            head = head.next
        if l1 == None:
            head.next = l2
        elif l2 == None:
            head.next = l1
        return first.next