From 93559cf1faaaf7e492c732696ea194cc7d2aa74c Mon Sep 17 00:00:00 2001
From: Nikson2003 <164474952+Nikson2003@users.noreply.github.com>
Date: Sat, 5 Oct 2024 09:50:17 +0530
Subject: [PATCH] Create Reverse Nodes in k-Group (LeetCode 25)

This is a LeetCode Hard Problem (25) under the LinkedList data structure.
---
 .../Reverse Nodes in k-Group (LeetCode 25)    | 45 +++++++++++++++++++
 1 file changed, 45 insertions(+)
 create mode 100644 linked_list/Reverse Nodes in k-Group (LeetCode 25)

diff --git a/linked_list/Reverse Nodes in k-Group (LeetCode 25) b/linked_list/Reverse Nodes in k-Group (LeetCode 25)
new file mode 100644
index 00000000..8ee151dd
--- /dev/null
+++ b/linked_list/Reverse Nodes in k-Group (LeetCode 25)	
@@ -0,0 +1,45 @@
+/**
+ * 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 reverseKGroup(ListNode head, int k) {
+        ListNode dummy = new ListNode(0, head);  // create a dummy node to return its next (head) at the end
+        ListNode prev = dummy;
+        while (true) {
+            ListNode tail = prev;
+            int count = 0;
+            while (tail != null && count < k) {
+                tail = tail.next;
+                count ++;
+            }
+            if(tail == null) {  // break if the current node (tail) becomes null
+                break;
+            }
+            ListNode start = prev.next;
+            ListNode nextStart = tail.next;
+            prev.next = reverseList(start, tail);
+            prev = start;
+        }
+        return dummy.next;
+    }
+
+    private ListNode reverseList(ListNode start, ListNode end) {
+        ListNode prev = end.next;
+        while (start != end) {
+            ListNode next = start.next;
+            start.next = prev;
+            prev = start;
+            start = next;
+        }
+        start.next = prev;
+        
+        return end;
+    }
+}