此题建议画图, 绝对要画图, 画完都不太用想的

代码

class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode dummy = new ListNode();
        dummy.next = head;

        ListNode A = dummy;
        ListNode B = head;
        while(B != null && B.next != null){
            ListNode C = B.next;    
            A.next = B.next;
            B.next = C.next;
            C.next = B;

            A = B;
            B = B.next;
        }

        return dummy.next;
    }
}