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.

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

Python

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class ListNode:
def __init__(self, x):
self.val = x
self.next = None
class Solution:
def swapPairs(self, head):
if head == None:
return None
t = ListNode(0)
t.next = head
head = t
while t is not None and t.next is not None and t.next.next is not None:
a = t.next
t.next = a.next
a.next = t.next.next
t.next.next = a
t = a
return head.next