思路:

• 根据题意画图
• 分析出要两两交换节点首先头节点会被交换，所以引入dummy
• 再一个分析得出需要三个指针 pre cur next
• 再根据画图来分析三个指针的交换次序

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        if(head == null || head.next == null)
            return head;
        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        ListNode pre = dummy;
        ListNode cur = head;
        while(cur != null && cur.next != null){
            ListNode next = cur.next;
            cur.next = next.next;
            next.next = cur;
            pre.next = next;
            pre = cur;
            cur = pre.next;
        }
        return dummy.next;
    }
}