LeetCode-Swap Nodes in Pairs

题目：https://oj.leetcode.com/problems/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.
源码：Java版本

算法分析：时间复杂度O(n)，空间复杂度O(1)

/**
 * 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 newHead=new ListNode(0);
        ListNode cur=head,pre=newHead,next,nextnext;
        while(cur!=null) {
            next=cur.next;
            cur.next=null;
            if(next==null) {
                pre.next=cur;
                break;
            }
            nextnext=next.next;
            next.next=cur;
            pre.next=next;
            pre=cur;
            cur=nextnext;
        }
        return newHead.next;
    }
}