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
# 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
"""
mhead=ListNode(-1)
pre =mhead
while l1 and l2:
if l1.val<l2.val:
pre.next=l1
l1=l1.next
else:
pre.next=l2
l2=l2.next
pre=pre.next
if l1 is None:
pre.next=l2
elif l2 is None:
pre.next=l1
return mhead.next