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方法一 递归

/** * 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 newHead = head.next;        head.next = swapPairs(newHead.next);        newHead.next = head;        return newHead;    }}

方法二 迭代

class Solution {
       public ListNode swapPairs(ListNode head) {
           ListNode dummyHead = new ListNode(0);        dummyHead.next = head;        ListNode temp = dummyHead;        while (temp.next != null && temp.next.next != null) {
               ListNode node1 = temp.next;            ListNode node2 = temp.next.next;            temp.next = node2;            node1.next = node2.next;            node2.next = node1;            temp = node1;        }        return dummyHead.next;    }}

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