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前言

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遞歸寫法和雙指針法實質上都是從前往后翻轉指針指向，其實還有另外一種與雙指針法不同思路的遞歸寫法：從后往前翻轉指針指向。

一、力扣206. 反轉鏈表

/*** 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 reverseList(ListNode head) {ListNode p = new ListNode();ListNode r = head; p.next = null;while(r != null){ListNode s = r;r = r.next;s.next = p.next;p.next = s;}return p.next;}
}

二、力扣24. 兩兩交換鏈表中的節點

/*** 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) {ListNode L = new ListNode(-1,head);if(head == null || head.next == null){return head;}ListNode p1 = L, p2 = p1.next, p3 = p2.next;while(p2 != null && p3 != null){ListNode s1 = p2, s2 = p3;p2 = p3.next;if(p2 != null){p3 = p2.next;}else{p3 = null;}p1.next = s2;s2.next = s1;s1.next = p2;p1 = s1;}return L.next;}
}