1+ // 19. 删除链表的倒数第 N 个结点
2+
3+
4+ /**
5+ * Definition for singly-linked list.
6+ * public class ListNode {
7+ * int val;
8+ * ListNode next;
9+ * ListNode() {}
10+ * ListNode(int val) { this.val = val; }
11+ * ListNode(int val, ListNode next) { this.val = val; this.next = next; }
12+ * }
13+ */
14+
15+
16+ /*
17+ 快慢指针：快指针先走n步，然后快慢指针同时走，直到快指针走完链表，此时慢指针所指节点的下一节点是待删除节点，修改慢指针所指节点的下一节点指针指向为下下节点
18+ */
19+ class Solution {
20+ public ListNode removeNthFromEnd (ListNode head , int n ) {
21+ ListNode root = new ListNode (0 , head );
22+ ListNode slow = root ;
23+ ListNode fast = head ;
24+ for (int i = 0 ; i < n ; i ++) {
25+ fast = fast .next ;
26+ }
27+ while (fast != null ) {
28+ fast = fast .next ;
29+ slow = slow .next ;
30+ }
31+ slow .next = slow .next .next ;
32+ return root .next ;
33+ }
34+ }
35+
36+
37+ /*
38+ 栈：后进先出，全部入栈后弹出n个节点，第n个为待删除节点，此时栈顶节点为前驱节点，修改前驱节点的下一节点指针指向下下节点
39+ */
40+ class Solution {
41+ public ListNode removeNthFromEnd (ListNode head , int n ) {
42+ Stack <ListNode > stack = new Stack <>();
43+ ListNode root = new ListNode (0 , head );
44+ ListNode cur = root ;
45+ while (cur != null ) {
46+ stack .push (cur );
47+ cur = cur .next ;
48+ }
49+ while (n > 0 ) {
50+ stack .pop ();
51+ n --;
52+ }
53+ ListNode node = stack .peek ();
54+ node .next = node .next .next ;
55+ return root .next ;
56+ }
57+ }
58+
59+
60+ /*
61+ 计数：遍历链表记录链表长度，计算待删除节点正数位置，再从头遍历到待删除节点的前驱节点位置，修改前驱节点的下一节点指针指向下下节点
62+ */
63+ class Solution {
64+ public ListNode removeNthFromEnd (ListNode head , int n ) {
65+ ListNode root = new ListNode (0 , head );
66+ ListNode cur = root ;
67+ int len = 0 ;
68+ while (head != null ) {
69+ len ++;
70+ head = head .next ;
71+ }
72+ for (int i = 0 ; i < len - n ; i ++) {
73+ cur = cur .next ;
74+ }
75+ cur .next = cur .next .next ;
76+ return root .next ;
77+ }
78+ }
79+
80+
81+ /*
82+ 递归：累加计数
83+ 1、方法功能：传入节点，返回该节点的倒数位置个数。如果是目标删除位置，则修改链表指针指向下下个节点
84+ 2、终止条件：节点为空时，返回位置0
85+ 3、递归逻辑：
86+ 1）调用递归方法，直接走到链表尾部，逐层累加计数返回
87+ 2）接收递归返回值，判断该值是否为目标删除位置，是则说明下一节点是待删除节点，修改链表指针指向下下个节点
88+ */
89+ class Solution {
90+ public ListNode removeNthFromEnd (ListNode head , int n ) {
91+ return rec (head , n ) == n ? head .next : head ;
92+ }
93+
94+ private int rec (ListNode head , int n ) {
95+ if (head == null ) {
96+ return 0 ;
97+ }
98+ int num = rec (head .next , n );
99+ if (num == n ) {
100+ head .next = head .next .next ;
101+ }
102+ return ++num ;
103+ }
104+ }
105+
106+
107+ /*
108+ 递归：递减计数
109+ */
110+ class Solution {
111+ public ListNode removeNthFromEnd (ListNode head , int n ) {
112+ return track (head , n ) == 0 ? head .next : head ;
113+ }
114+
115+ private int track (ListNode head , int num ) {
116+ if (head == null ) {
117+ return num ;
118+ }
119+ num = track (head .next , num );
120+ if (num == 0 ) {
121+ head .next = head .next .next ;
122+ }
123+ return --num ;
124+ }
125+ }
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