Day 23 两两交换链表

没看懂，只能先抄了再说

class ListNode:
def init(self, x):
self.val = x
self.next = None

class Solution:
def swapPairs(self, head):
if head is None or head.next is None:
return head
tmp = head.next
r = self.swapPairs(tmp.next)
tmp.next = head
head.next = r
return tmp

if name == 'main':
a = [1,2,3,4,5]
head = ListNode(a[0])

tmp = head
for i in a[1:]:
    node = ListNode(i)
    tmp.next = node
    tmp = tmp.next

tmp = head
while tmp:
    print(tmp.val)
    tmp = tmp.next

shead = Solution().swapPairs(head)
tmp = shead
while tmp:
    print(tmp.val)
    tmp = tmp.next