问题的思路是这样：
循环取头部合并，其实也可以换个角度来看，就是将后面的链表结点，一次隔空插入到第一部分的链表中。
class Solution:
# @param head, a ListNode
# @return nothing
def reorderList(self, head):
if None == head or None == head.next:
return head
#找到中间点，截断
pfast = head
pslow = head
while pfast.next and pfast.next.next:
pfast = pfast.next.next
pslow = pslow.next
pfast = pslow.next
pslow.next = None
#将第二部分的结点逆置
pnext = pfast.next
pfast.next = None
while pnext:
q = pnext.next
pnext.next = pfast
pfast = pnext
pnext = q
#将逆置后的链表隔空插入到第一部分
tail = head
while pfast:
pnext = pfast.next
pfast.next = tail.next
tail.next = pfast
tail = tail.next.next
pfast = pnext
return head