题目：
Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.
代码：
def mergeTwoLists(self, l1, l2):
"""
:type l1: ListNode
:type l2: ListNode
:rtype: ListNode
"""
head = ListNode(None)
p = head
while l1!=None and l2!=None:
p.next = l1
if l1.val < l2.val:
p = p.next
l1 = l1.next
else:
p.next = l2
p = p.next
l2 = l2.next
if l1 != None:
p.next = l1
if l2 != None:
p.next = l2
return head.next