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 = Noneclass 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