LeetCode之旅（21）-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：

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) {
if(head == null||head.next == null){
return head;
}
ListNode first = head;
ListNode second = head.next;
ListNode pre = null;
ListNode swap = null;
if(second != null){
head = second;
}
while(first != null&&second != null){
swap = second.next;
second.next = first;
first.next = swap;
if(pre != null){
pre.next = second;
}
pre = first;
first = swap;
if(swap != null){
second = swap.next;
}
}
return head;
}
}