反转k个节点的k==2版本

class ListNode {
    int val;
    ListNode next;
    ListNode() {}
    ListNode(int val) { this.val = val; }
    ListNode(int val, ListNode next) { this.val = val; this.next = next; }
}
// @lc code=start
/**
 * 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 {
    /**
     * 反转k个节点的k==2版本
     */
    public ListNode swapPairs(ListNode head) {
        ListNode dummy = new ListNode(0, head);
        ListNode p0 = dummy;
        while (head != null && head.next != null) {
            ListNode nxt = head.next.next;
            head.next.next = head;
            p0.next = head.next;
            head.next = nxt;
            p0 = head;
            head = nxt;
        }
        return dummy.next;
    }
}