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。
别用递归，递归会慢。
其他没啥了，挺简单的。
class Solution:
def mergeTwoLists(self, l1, l2):
"""
:type l1: ListNode
:type l2: ListNode
:rtype: ListNode
"""
head = ListNode(0)
first = head
while l1 is not None and l2 is not None:
if l1.val <= l2.val:
head.next = l1
l1 = l1.next
else:
head.next = l2
l2 = l2.next
head = head.next
if l1 is not None:
head.next = l1
if l2 is not None:
head.next = l2
return first.next
原文：https://www.cnblogs.com/coder-chi/p/10197119.html