Java Program For Reversing A Linked List In Groups Of Given Size- Set 2 - GeeksforGeeks

11 Jan, 2022
Given a linked list, write a function to reverse every k nodes (where k is an input to the function). Examples:
Input: 1->2->3->4->5->6->7->8->NULL and k = 3 
Output: 3->2->1->6->5->4->8->7->NULL. 

Input: 1->2->3->4->5->6->7->8->NULL and k = 5
Output: 5->4->3->2->1->8->7->6->NULL.
We have already discussed its solution in below post Reverse a Linked List in groups of given size | Set 1In this post, we have used a stack which will store the nodes of the given linked list. Firstly, push the k elements of the linked list in the stack. Now pop elements one by one and keep track of the previously popped node. Point the next pointer of prev node to top element of stack. Repeat this process, until NULL is reached.This algorithm uses O(k) extra space.
Java
// Java program to reverse a linked list // in groups of given size import java.util.*;class GfG {    // Link list node     static class Node     {         int data;         Node next;     }    static Node head = null;      /* Reverses the linked list in groups        of size k and returns the pointer        to the new head node. */    static Node Reverse(Node head, int k)     {         // Create a stack of Node*         Stack<Node> mystack =                     new Stack<Node> ();         Node current = head;         Node prev = null;           while (current != null)        {            // Terminate the loop whichever             // comes first either current == NULL             // or count >= k             int count = 0;             while (current != null && count < k)            {                 mystack.push(current);                 current = current.next;                 count++;             }               // Now pop the elements of stack             // one by one             while (mystack.size() > 0)             {                 // If final list has not been                 // started yet.                 if (prev == null)                {                     prev = mystack.peek();                     head = prev;                     mystack.pop();                 }                 else                {                     prev.next = mystack.peek();                     prev = prev.next;                     mystack.pop();                 }             }         }           // Next of last element will point        // to NULL.         prev.next = null;           return head;     }       // UTILITY FUNCTIONS     // Function to push a node     static void push( int new_data)     {         // Allocate node         Node new_node = new Node();           // Put in the data         new_node.data = new_data;           // Link the old list off the         // new node         new_node.next = head;           // Move the head to point to         // the new node         head = new_node;     }       // Function to print linked list     static void printList(Node node)     {         while (node != null)         {             System.out.print(node.data + " ");             node = node.next;         }     }       // Driver code    public static void main(String[] args)     {         // Start with the empty list         //Node head = null;           /* Created Linked list is            1->2->3->4->5->6->7->8->9 */        push( 9);         push( 8);         push( 7);         push( 6);         push( 5);         push(4);         push(3);         push(2);         push( 1);           System.out.println(               "Given linked list ");         printList(head);         head = Reverse(head, 3);        System.out.println();          System.out.println(               "Reversed Linked list ");         printList(head);     } } // This code is contributed by Prerna Saini
Output: 
Given Linked List
1 2 3 4 5 6 7 8 9 
Reversed list
3 2 1 6 5 4 9 8 7
Please refer complete article on Reverse a Linked List in groups of given size | Set 2 for more details!
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