[Leetcode]Swap Nodes in Pairs

Given a linked list, swap every two adjacent nodes and return its head.

For example,

Given 

1->2->3->4

, you should return the list as 

2->1->4->3

.

Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

链表题  每两个节点翻转一次

class Solution:
# @param a ListNode
# @return a ListNode
def swapPairs(self, head):
if head is None or head.next is None: return head
dummy = ListNode(0)
dummy.next = head
pre, curr = dummy, head
while curr != None and curr.next != None:
tmp = curr.next
pre.next = tmp
curr.next = tmp.next
tmp.next = curr
pre = curr; curr = curr.next
return dummy.next