链表问题---合并两个有序的单链表

#python3.5
def merge(head1, head2):
if head1 == None or head2 == None:
return head1 if head2 == None else head2
head = head1 if head1.val < head2.val else head2
cur1 = head1 if head == head1 else head2
cur2 = head1 if head == head2 else head2
pre = None
while cur1 != None and cur2 != None:
if cur1.val <= cur2.val:
pre = cur1
cur1 = cur1.next
else:
next = cur2.next
pre.next = cur2
cur2.next = cur1
pre = cur2
cur2 = next
pre.next = cur1 if cur2 == None else cur2
return head