24. Swap Nodes in Pairs [easy]

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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.

Java

/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { *         val = x; *         next = null; *     } * } */public class Solution {    public ListNode swapPairs(ListNode head) {        if(head==null||head.next==null) return head;        ListNode pre = new ListNode(0); ListNode dummy = pre, cur = head, post = cur.next;        pre.next = head;        while(post!=null){            cur.next = post.next;            post.next = cur;            pre.next = post;            pre = cur;            cur = cur.next;            if(cur==null) break;            post = cur.next;        }        return dummy.next;    }}

Python

# Definition for singly-linked list.# class ListNode(object):#     def __init__(self, x):#         self.val = x#         self.next = Noneclass Solution(object):    def swapPairs(self, head):        """        :type head: ListNode        :rtype: ListNode        """        dup=ListNode(0); dup.next=head;p=dup        while p.next!=None and p.next.next!=None:            temp=p.next            p.next=temp.next            p=p.next            temp.next=p.next            p.next=temp            #关键            p=temp        return dup.next

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