问题描述：
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
方法1：
class Solution(object):
def mergeTwoLists(self, l1, l2):
"""
:type l1: ListNode
:type l2: ListNode
:rtype: ListNode
"""
p = dummy = ListNode(-1)
while l1 and l2:
if l1.val <= l2.val:
p.next = ListNode(l1.val)
l1 = l1.next
else:
p.next = ListNode(l2.val)
l2 = l2.next
p = p.next
p.next = l1 or l2
return dummy.next
方法2：递归
class Solution(object):
def mergeTwoLists(self, l1, l2):
"""
:type l1: ListNode
:type l2: ListNode
:rtype: ListNode
"""
if l1 == None:
return l2
if l2 == None:
return l1
mergel = None #设置新链表
if l1.val < l2.val:
mergel = l1
mergel.next = self.mergeTwoLists(l1.next, l2)
else:
mergel = l2
mergel.next = self.mergeTwoLists(l1, l2.next)
return mergel