解题思路
这道题用画图的方法是比较好的。

相关代码

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode H = new ListNode(0);
        H.next = head;
        ListNode p1 = H;
        ListNode p2 = H.next;
        if(p2==null){
            return H.next;
        }
        ListNode p3 = H.next.next;
        
        while(p1!=null&&p2!=null&&p3!=null){
            ListNode p4 = p3.next;
            p1.next = p3;
            p3.next = p2;
            p2.next = p4;
            ListNode t1 = p2;
            ListNode t2 = p4;
            if(p4 == null){
                break;
            }
            ListNode t3 = p4.next;
            p1 = t1;
            p2 = t2;
            p3 = t3;
        }
        return H.next;            
    }
}