Swap Nodes in Pairs leetcode

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.
每次操作两个节点, 并且要记录两个节点前边的一个节点prev
例如1-->2-->3-->4, 对于每次循环
对于3,4 节点 把3放到4后面, 然后把prev 2 指向4, 再记录3节点为prev

时间O(n) 空间O(1)

public class Solution {
    public ListNode swapPairs(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode pre = dummy;
        ListNode node = head;
        while (node != null && node.next != null) {
            ListNode tem = node.next;
            node.next = node.next.next;
            tem.next = node;
            pre.next = tem;
            pre = node;
            node = node.next;
        }
        return dummy.next;
    }
}