24. Swap Nodes in Pairs

**问题描述：

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.**

下面附上代码：

/**
* 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 pre = new ListNode(0);
pre = head;
while(pre!=null){
int temp = pre.val;
if(pre.next!=null){
pre.val = pre.next.val;
pre.next.val = temp;
}
else{
break;
}
pre=pre.next.next;
}
return head;
}
}