LeetCode Swap Nodes in Pairs

原题链接在这里：https://leetcode.com/problems/swap-nodes-in-pairs/

需要建一个dunmy，最后return dunmy.next. 如此可以避免边界问题的讨论，例如头两个点的转换。

AC Java:

/**
 * 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 dunmy = new ListNode(0);
        dunmy.next = head;
        ListNode mark = dunmy;
        ListNode temp;
        ListNode cur;
        while(mark.next != null && mark.next.next!=null){
            cur = mark.next.next;
            temp = cur.next;
            mark.next.next=temp;
            cur.next = mark.next;
            mark.next = cur;
            mark = cur.next;
        }
        return dunmy.next;
    }
}