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Commit 54c8a48

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添加第64题
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/*
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* @lc app=leetcode id=92 lang=cpp
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*
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* [92] Reverse Linked List II
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*/
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// @lc code=start
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/**
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* Definition for singly-linked list.
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* struct ListNode {
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* int val;
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* ListNode *next;
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* ListNode() : val(0), next(nullptr) {}
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* ListNode(int x) : val(x), next(nullptr) {}
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* ListNode(int x, ListNode *next) : val(x), next(next) {}
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* };
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*/
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class Solution {
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public:
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// 递归法
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ListNode* reverseBetween(ListNode* head, int left, int right) {
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if (left == 1)
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// 如果从表头开始反转,相当于反转前N个节点
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return reverseN(head, right);
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// 对于 head->next 相当于反转区间为 [left - 1, right - 1]
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head->next = reverseBetween(head->next, left - 1, right - 1);
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return head;
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}
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ListNode* right_plus;
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ListNode* reverseN(ListNode* head, int right) {
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if (right == 1)
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{
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right_plus = head->next;
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return head;
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}
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ListNode* last = reverseN(head->next, right - 1);
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head->next->next = head;
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// head->next = nullptr; // 全链表翻转时
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head->next = right_plus;
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return last;
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}
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// 头插法
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ListNode* reverseBetween2(ListNode* head, int left, int right) {
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ListNode *dummy;
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dummy->next = head;
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ListNode *g = dummy, *p = dummy->next;
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for (size_t i = 0; i < left; i++)
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{
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g = g->next;
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p = p->next;
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}
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for (size_t i = 0; i < right - left; i++)
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{
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ListNode *tmp = p->next;
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p->next = p->next->next;
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tmp->next = g->next;
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g->next = tmp;
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}
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return dummy->next;
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}
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};
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// @lc code=end
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# 反转链表 II
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#### *Reverse Linked List II*
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给你单链表的头指针 `head` 和两个整数 `left` 和 `right` ,其中 `left <= right` 。请你反转从位置 `left` 到位置 `right` 的链表节点,返回 反转后的链表 。
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提示:
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给定链表的结点数介于 1 和 100 之间。
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英文题目:
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Given the `head` of a singly linked list and two integers `left` and `right` where `left <= right`, reverse the nodes of the list from position `left` to position `right`, and return *the reversed list*.
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**Constraints:**
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- The number of nodes in the list is `n`.
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- `1 <= n <= 500`
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- `-500 <= Node.val <= 500`
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- `1 <= left <= right <= n`
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**example 1**
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![example 1](https://github.com/SherlockUnknowEn/leetcode/blob/master/60-69/64.%20Reverse%20Linked%20List%20II(Medium)/rev2ex2.jpg)
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```
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Input: head = [1,2,3,4,5], left = 2, right = 4
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Output: [1,4,3,2,5]
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```
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**example 2**
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```
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Input: head = [5], left = 1, right = 1
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Output: [5]
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```
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---
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### 思路
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1. 递归法:为`head`节点编号为1时,反转`[left, right]`之间的节点。当第二个节点(`head->next`)编号为1时,题目相当于反转`[left-1, right-1]`之间的节点。当第`left`个节点编号为1时,题目相当于反转**前`right - left`个**节点。
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2. 反转前`N`个节点的算法,由`reverseN(ListNode* head, int N)`函数给出
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3. 头插法:设`g`为`left`左边的节点,`p`从`left`节点移动到`right`,`p`遍历过程使用**头插法**插入到`g`后,即可实现倒序
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### 代码
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```cpp
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/*
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* @lc app=leetcode id=92 lang=cpp
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*
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* [92] Reverse Linked List II
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*/
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// @lc code=start
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/**
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* Definition for singly-linked list.
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* struct ListNode {
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* int val;
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* ListNode *next;
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* ListNode() : val(0), next(nullptr) {}
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* ListNode(int x) : val(x), next(nullptr) {}
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* ListNode(int x, ListNode *next) : val(x), next(next) {}
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* };
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*/
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class Solution {
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public:
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// 递归法
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ListNode* reverseBetween(ListNode* head, int left, int right) {
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if (left == 1)
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// 如果从表头开始反转,相当于反转前N个节点
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return reverseN(head, right);
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// 对于 head->next 相当于反转区间为 [left - 1, right - 1]
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head->next = reverseBetween(head->next, left - 1, right - 1);
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return head;
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}
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ListNode* N1_node;
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ListNode* reverseN(ListNode* head, int N) {
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if (N == 1)
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{
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N1_node = head->next;
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return head;
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}
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ListNode* last = reverseN(head->next, N - 1);
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head->next->next = head;
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// head->next = nullptr; // 全链表翻转时,原先的头节点变为尾节点,next指向null
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head->next = N1_node; // 反转前N个节点时,原先的头结点next指向第 N+1 个
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return last;
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}
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// 头插法
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ListNode* reverseBetween2(ListNode* head, int left, int right) {
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ListNode *dummy;
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dummy->next = head;
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ListNode *g = dummy, *p = dummy->next;
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for (size_t i = 0; i < left; i++)
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{
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g = g->next;
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p = p->next;
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}
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for (size_t i = 0; i < right - left; i++)
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{
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ListNode *tmp = p->next;
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p->next = p->next->next;
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tmp->next = g->next;
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g->next = tmp;
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}
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return dummy->next;
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}
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};
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// @lc code=end
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```
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本题以及其它leetcode题目代码github地址: [github地址](https:github.com/SherlockUnknowEn/leetcode)
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‎README.md‎

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##### 61. [142.Linked List Cycle II](https://github.com/SherlockUnknowEn/leetcode/tree/master/60-69/61.%20Linked%20List%20Cycle%20II(Medium)) 环形链表 II
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##### 62. [160.Intersection of Two Linked Lists](https://github.com/SherlockUnknowEn/leetcode/tree/master/60-69/62.%20Intersection%20of%20Two%20Linked%20Lists(Easy)) 相交链表
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##### 63. [876.Middle of the Linked List](https://github.com/SherlockUnknowEn/leetcode/tree/master/60-69/63.%20Middle%20of%20the%20Linked%20List(Easy)) 链表的中间结点
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##### 64. [92.Reverse Linked List II](https://github.com/SherlockUnknowEn/leetcode/tree/master/60-69/64.%20Reverse%20Linked%20List%20II(Medium)) 反转链表 II
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