Reverse Alternate K group nodes

Author: ParthDhavan04

Linked List/ReverseAlternateKgroup.java

@@ -0,0 +1,42 @@
+//reverse alternate k group nodes in LL
+// https://www.geeksforgeeks.org/reverse-alternate-k-nodes-in-a-singly-linked-list/
+public ListNode reverseAlternateKGroup(ListNode head, int k) {
+        if (k <= 1 || head == null) {
+            return head;
+        }
+
+        // skip the first left-1 nodes
+        ListNode current = head;
+        ListNode prev = null;
+
+        while (current != null) {
+            ListNode last = prev;
+            ListNode newEnd = current;
+
+            // reverse between left and right
+            ListNode next = current.next;
+            for (int i = 0; current != null && i < k; i++) {
+                current.next = prev;
+                prev = current;
+                current = next;
+                if (next != null) {
+                    next = next.next;
+                }
+            }
+
+            if (last != null) {
+                last.next = prev;
+            } else {
+                head = prev;
+            }
+
+            newEnd.next = current;
+
+            // skip the k nodes
+            for (int i = 0; current != null && i < k; i++) {
+                prev = current;
+                current = current.next;
+            }
+        }
+        return head;
+    }