solution(java): 25. Reverse Nodes in k-Group

25. Reverse Nodes in k-Group
- Java

.gitignore

@@ -1,2 +1,3 @@
 directories.ipynb
-.vscode
+.vscode/
+lib/

Hard/25. Reverse Nodes in k-Group/ListNode.java

@@ -0,0 +1,72 @@
+//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){
+        //Edge cases when list is null or has less than one node
+        if(k <= 1 && head == null || head.next == null){
+            return head;
+        }        
+
+        //Start by getting the size of the list
+        ListNode temp = head;
+        int size = 0;
+
+        while(temp != null){
+            size++;
+            temp = temp.next;
+        }
+
+        //Initialize curr, temp and newhead pointers to have access points while traversing the list
+        ListNode curr = head;
+        ListNode prev = null;
+        ListNode newHead = null;
+
+        //Run a loop to reverse k group. Only get in this group if size is greater than k
+        while(size >= k){
+            //Initiallize nodes to keep track during reverse process
+            ListNode last = prev;
+            ListNode newEnd = curr;
+            ListNode next = curr.next;
+
+            //loop k times to reverse the k group
+            for (int i = 0; curr != null && i < k; i++) {
+                curr.next = prev;
+                prev = curr;
+                curr = next;
+
+                //Check to move next if it is not equal to null
+                if(next != null){
+                    next = next.next;
+                }
+            }
+
+            //Assign last node - node before the the beginning of the next sublist
+            if(last != null){
+                last.next = prev;
+            }else{
+                //Assign the newHead with prev
+                newHead = prev;
+            }
+
+            //Point the newEnd to the next node of the main list
+            newEnd.next = curr; 
+
+            //Reduce the size of the list by k times to a void a infinite loop
+            size -= k;
+
+            //Once the end of a part is done, assign prev with newEnd
+            prev = newEnd;
+        }
+        return newHead;          
+    }  
+ }
+
+