[leetcode] 24. 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.

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def swapPairs(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """

        link=ListNode(0)
        link.next=head
        tail=link
        while tail.next and tail.next.next:
            a=tail.next
            b=a.next
            tail.next,a.next,b.next=b,b.next,a
            tail=a

        return link.next  

Status: Accepted
Runtime: 44 ms
Ranking: beats 81.51%