[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