def swapPairs(self,head):
pre,pre.next = self,head
while pre.next and pre.next.next
a = pre.next
b = a.next
pre.next,b.next,a.next = b,a,b.next
pre = a
return self.next
def swapPairs(self,head):
pre,pre.next = self,head
while pre.next and pre.next.next
a = pre.next
b = a.next
pre.next,b.next,a.next = b,a,b.next
pre = a
return self.next