feat: Add LC 25 reverse nodes in k group (#156)

Author: snowan

README.en.md

@@ -216,6 +216,7 @@ The data structures mainly includes:
 
 - [0004.median-of-two-sorted-array](./problems/4.median-of-two-sorted-array.md) 🆕
 - [0023.merge-k-sorted-lists](./problems/23.merge-k-sorted-lists.md)
+- [0025.reverse-nodes-in-k-group](./problems/25.reverse-nodes-in-k-groups-en.md) 🆕
 - [0032.longest-valid-parentheses](./problems/32.longest-valid-parentheses.md) 🆕
 - [0042.trapping-rain-water](./problems/42.trapping-rain-water.md)
 - [0124.binary-tree-maximum-path-sum](./problems/124.binary-tree-maximum-path-sum.md)

README.md

@@ -220,6 +220,7 @@ leetcode 题解，记录自己的 leetcode 解题之路。
 
 - [0004.median-of-two-sorted-array](./problems/4.median-of-two-sorted-array.md) 🆕
 - [0023.merge-k-sorted-lists](./problems/23.merge-k-sorted-lists.md)
+- [0025.reverse-nodes-in-k-group](./problems/25.reverse-nodes-in-k-groups-cn.md) 🆕
 - [0032.longest-valid-parentheses](./problems/32.longest-valid-parentheses.md) 🆕
 - [0042.trapping-rain-water](./problems/42.trapping-rain-water.md)
 - [0124.binary-tree-maximum-path-sum](./problems/124.binary-tree-maximum-path-sum.md)

assets/problems/25.reverse-nodes-in-k-groups-1.PNG

@@ -0,0 +1,207 @@
+## 题目地址
+https://leetcode.com/problems/reverse-nodes-in-k-group/
+
+## 题目描述
+```
+Given a linked list, reverse the nodes of a linked list k at a time and return its modified list.
+
+k is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of k then left-out nodes in the end should remain as it is.
+
+Example:
+
+Given this linked list: 1->2->3->4->5
+
+For k = 2, you should return: 2->1->4->3->5
+
+For k = 3, you should return: 3->2->1->4->5
+
+Note:
+
+Only constant extra memory is allowed.
+You may not alter the values in the list's nodes, only nodes itself may be changed.
+
+```
+
+## 思路
+题意是以 `k` 个nodes为一组进行翻转，返回翻转后的`linked list`.
+
+从左往右扫描一遍`linked list`，扫描过程中，以k为单位把数组分成若干段，对每一段进行翻转。给定首尾nodes，如何对链表进行翻转。
+
+链表的翻转过程，初始化一个为`null `的 `previous node（prev）`，然后遍历链表的同时，当前`node （curr）`的下一个（next）指向前一个`node（prev）`，
+在改变当前node的指向之前，用一个临时变量记录当前node的下一个`node（curr.next)`. 即
+```
+ListNode temp = curr.next;
+curr.next = prev;
+prev = curr;
+curr = temp;
+```
+
+举例如图：翻转整个链表 `1->2->3->4->null` -> `4->3->2->1->null`
+
+![reverse linked list](../assets/problems/25.reverse-nodes-in-k-groups-1.PNG)
+
+这里是对每一组（`k个nodes`）进行翻转，
+
+1. 先分组，用一个`count`变量记录当前节点的个数
+
+2. 用一个`start` 变量记录当前分组的起始节点位置的前一个节点
+
+3. 用一个`end `变量记录要翻转的最后一个节点位置
+
+4. 翻转一组（`k个nodes`）即`(start, end) - start and end exclusively`。
+
+5. 翻转后，`start`指向翻转后链表, 区间`（start，end）`中的最后一个节点, 返回`start` 节点。
+
+6. 如果不需要翻转，`end` 就往后移动一个（`end=end.next`)，每一次移动，都要`count+1`.
+
+如图所示 步骤4和5： 翻转区间链表区间`（start， end）`
+
+![reverse linked list range in (start, end)](../assets/problems/25.reverse-nodes-in-k-groups-3.png)
+
+
+举例如图，`head=[1,2,3,4,5,6,7,8], k = 3`
+
+
+![reverse k nodes in linked list](../assets/problems/25.reverse-nodes-in-k-groups-2.PNG)
+
+
+>**NOTE**: 一般情况下对链表的操作，都有可能会引入一个新的`dummy node`，因为`head`有可能会改变。这里`head 从1->3`, 
+`dummy (List(0)) `保持不变。
+
+#### 复杂度分析
+- *时间复杂度:* `O(n) - n is number of Linked List`
+- *空间复杂度:* `O(1)`
+
+## 关键点分析
+1. 创建一个dummy node
+2. 对链表以k为单位进行分组，记录每一组的起始和最后节点位置
+3. 对每一组进行翻转，更换起始和最后的位置
+4. 返回`dummy.next`.
+
+## 代码 (`Java/Python3`)
+*Java Code*
+```java
+class ReverseKGroupsLinkedList {
+  public ListNode reverseKGroup(ListNode head, int k) {
+      if (head == null || k == 1) {
+        return head;
+      }
+      ListNode dummy = new ListNode(0);
+      dummy.next = head;
+  
+      ListNode start = dummy;
+      ListNode end = head;
+      int count = 0;
+      while (end != null) {
+        count++;
+        // group
+        if (count % k == 0) {
+          // reverse linked list (start, end]
+          start = reverse(start, end.next);
+          end = start.next;
+        } else {
+          end = end.next;
+        }
+      }
+      return dummy.next;
+    }
+  
+    /** 
+     * reverse linked list from range (start, end), return last node.
+     * for example: 
+     * 0->1->2->3->4->5->6->7->8
+     * |           |
+     * start       end
+     * 
+     * After call start = reverse(start, end)
+     * 
+     * 0->3->2->1->4->5->6->7->8
+     *          |  |
+     *       start end
+     *       first
+     * 
+     */
+    private ListNode reverse(ListNode start, ListNode end) {
+      ListNode curr = start.next;
+      ListNode prev = start;
+      ListNode first = curr;
+      while (curr != end){
+        ListNode temp = curr.next;
+        curr.next = prev;
+        prev = curr;
+        curr = temp;
+      }
+      start.next = prev;
+      first.next = curr;
+      return first;
+    }
+}
+```
+
+*Python3 Cose*
+```python
+class Solution:
+    def reverseKGroup(self, head: ListNode, k: int) -> ListNode:
+        if head is None or k < 2:
+            return head
+        dummy = ListNode(0)
+        dummy.next = head
+        start = dummy
+        end = head
+        count = 0
+        while end:
+            count += 1
+            if count % k == 0:
+                start = self.reverse(start, end.next)
+                end = start.next
+            else:
+                end = end.next
+        return dummy.next
+    
+    def reverse(self, start, end):
+        prev, curr = start, start.next
+        first = curr
+        while curr != end:
+            temp = curr.next
+            curr.next = prev
+            prev = curr
+            curr = temp
+        start.next = prev
+        first.next = curr
+        return first   
+```
+
+## 参考（References)
+- [Leetcode Discussion (yellowstone)](https://leetcode.com/problems/reverse-nodes-in-k-group/discuss/11440/Non-recursive-Java-solution-and-idea)
+
+## 扩展
+
+- 要求从后往前以`k`个为一组进行翻转。**(字节跳动（ByteDance）面试题)**
+
+    例子，`1->2->3->4->5->6->7->8, k = 3`, 
+    
+    从后往前以`k=3`为一组，
+    - `6->7->8` 为一组翻转为`8->7->6`， 
+    - `3->4->5`为一组翻转为`5->4->3`. 
+    - `1->2`只有2个nodes少于`k=3`个，不翻转。
+    
+    最后返回： `1->2->5->4->3->8->7->6`
+
+这里的思路跟从前往后以`k`个为一组进行翻转类似，可以进行预处理：
+
+1. 翻转链表
+
+2. 对翻转后的链表进行从前往后以k为一组翻转。
+
+3. 翻转步骤2中得到的链表。
+
+例子：`1->2->3->4->5->6->7->8, k = 3`
+
+1. 翻转链表得到：`8->7->6->5->4->3->2->1`
+
+2. 以k为一组翻转： `6->7->8->3->4->5->2->1`
+
+3. 翻转步骤#2链表： `1->2->5->4->3->8->7->6`
+
+## 类似题目
+- [Swap Nodes in Pairs](https://leetcode.com/problems/swap-nodes-in-pairs/)