package org.example;

public class SwapPairs {

public ListNode swapPairs(ListNode head) {

if (head == null) {

return null;

}

ListNode dummy = new ListNode();

dummy.next = head;

exchange(head,dummy);

return dummy.next;

}

private ListNode exchange(ListNode head, ListNode pre) {

if (head == null) {

return null;

}

if (head.next == null) {

return head;

}

ListNode first = head;

ListNode second = head.next;

ListNode third = second.next;

pre.next = second;

second.next = first;

first.next = third;

pre = first;

exchange(third,pre);

return second;

}

}