// @Title: 两两交换链表中的节点 (Swap Nodes in Pairs)
// @Author: qisiii
// @Date: 2024-09-12 00:41:34
// @Runtime: 0 ms
// @Memory: 40.3 MB
// @comment: 
// @flag: 
/**
 * 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) {
        if (head == null || head.next==null) {
            return head;
        }
        ListNode next = head.next;
        head.next = swapPairs(next != null ? next.next : null);
        if (next != null) {
            next.next = head;
        }
        return next;
    }
}

// @Title: 两两交换链表中的节点 (Swap Nodes in Pairs)
// @Author: qisiii
// @Date: 2024-04-13 21:13:33
// @Runtime: 0 ms
// @Memory: 40.2 MB
// @comment: 官方题解
// @flag: WHITE
/**
 * 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) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode temp = head.next;
        head.next=swapPairs(head.next.next);
        temp.next=head;
        return temp;
    }
}

// @Title: 两两交换链表中的节点 (Swap Nodes in Pairs)
// @Author: qisiii
// @Date: 2024-04-13 21:09:02
// @Runtime: 0 ms
// @Memory: 40.5 MB
// @comment: 借用临时的头
// @flag: WHITE
/**
 * 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) {
        if (head == null || head.next == null) {
            return head;
        }
        //-1->2
        ListNode temp = new ListNode(-1, head.next);
        //临时存储3
        ListNode next = head.next.next;
        //断开2和3
        head.next.next = null;
        //断开1和2
        head.next=null;
        temp.next.next = head;
        head.next = swapPairs(next);
        return temp.next;
    }
}