24. Swap Nodes in Pairs

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.

思路

首先判断是否可以取到2个节点，如果可以，对这两个节点的next进行交换，同时存储交换后的第二个节点为pre，下一轮取两个节点，当交换完后，把第一个节点赋给pre.next，保证上一步和下一步链接好

Python实现

# 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
        """
        if head==None or head.next==None:
            return head
        h=head.next
        pre=None
        while(head!=None and head.next!=None):
            p1=head
            p2=head.next
            temp=p1
            p1.next=p2.next
            p2.next=temp
            head=p1.next
            if pre!=None:pre.next=p2
            pre=p1
        return h