题目：
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.
代码：
def swapPairs(self, head):
"""
:type head: ListNode
:rtype: ListNode
"""
pre = None
p1 = head
while p1!=None and p1.next!=None:
p2 = p1.next
t = p2.next
p2.next = p1
p1.next = t
if pre == None:
head = p2
else:
pre.next = p2
pre = p1
p1 = p1.next
return head
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