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.

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/**
* Definition for singly-linked list.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode swapPairs(ListNode head) {
ListNode first = null;
ListNode second = null;
ListNode tmp = null;
ListNode ret = head;
first = head;
if(first==null)return head;
if(first.next==null)return head;
ret = head.next;
for(;first!=null&&first.next!=null;first=first.next){
second = first.next;
if(tmp!=null)tmp.next=second;
tmp = first;
first.next=second.next;
second.next=first;
}
return ret;
}
}