24. Swap Nodes in Pairs(成对交换链表节点)Python

题目：

给定一个单链表，成对交换相邻的两个节点，并返回链表头。

例如：

代码：

# 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
        """
        pre = new_head = ListNode(0)
        while (head!=None and head.next!=None):
            head_next = head.next
            head_next_next = head.next.next
            new_head.next = head_next
            head_next.next = head
            new_head = head_next.next
            head = head_next_next
        if head!=None:
            new_head.next = head
        else:
            new_head.next = None
        return pre.next