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How to reverse a linked list in pairs

Golu Rawat answered on August 24, 2022 Popularity 5/10 Helpfulness 1/10

answer How to reverse a linked list in pairs

How to reverse a linked list in pairs

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import java.util.*;
  
// Linked List Node
class Node {
    int data;
    Node next;
    Node(int a)
    {
        data = a;
        next = null;
    }
}
  
class GFG {
    // utility function to insert node in the list
    static Node push(Node head, int val)
    {
        Node newNode = new Node(val);
        if (head == null) {
            head = newNode;
            return head;
        }
  
        Node temp = head;
        while (temp.next != null)
            temp = temp.next;
  
        temp.next = newNode;
        return head;
    }
  
    // utility function to reverse k nodes in the list
    static Node reverse(Node head, int k)
    {
        // If head is NULL or K is 1 then return head
        if (head == null || head.next == null)
            return head;
  
        // creating dummy node
        Node dummy = new Node(-1);
        dummy.next = head;
  
        // Initializing three points prev, curr, next
        Node prev = dummy;
        Node curr = dummy;
        Node next = dummy;
  
        // Calculating the length of linked list
        int count = 0;
        while (curr != null) {
            count++;
            curr = curr.next;
        }
  
        // Iterating till next is not NULL
        while (next != null) {
            curr = prev.next; // Curr position after every
                              // reverse group
            next = curr.next; // Next will always next to
                              // curr
            int toLoop
                = count > k
                      ? k
                      : count - 1; // toLoop will set to
                                   // count - 1 in case of
                                   // remaining element
  
            for (int i = 1; i < toLoop; i++) {
                // 4 steps as discussed above
                curr.next = next.next;
                next.next = prev.next;
                prev.next = next;
                next = curr.next;
            }
            prev = curr; // Setting prev to curr
            count -= k; // Update count
        }
        return dummy.next; // dummy -> next will be our new
                           // head for output linked
        // list
    }
    // utility function to print the list
    static void print(Node head)
    {
        while (head.next != null) {
            System.out.print(head.data + " ");
            head = head.next;
        }
        System.out.println(head.data);
    }
  
    public static void main(String args[])
    {
        Node head = null;
        int k = 3;
        head = push(head, 1);
        head = push(head, 2);
        head = push(head, 3);
        head = push(head, 4);
        head = push(head, 5);
        head = push(head, 6);
        head = push(head, 7);
        head = push(head, 8);
        head = push(head, 9);
  
        System.out.println("Given Linked List");
        print(head);
        System.out.println("Reversed list");
        Node newHead = reverse(head, k);
        print(newHead);
    }
}

Popularity 5/10 Helpfulness 1/10 Language whatever

Source: www.geeksforgeeks.org

Tags: linked-list reverse whatever

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Contributed on Aug 24 2022

Golu Rawat

0 Answers Avg Quality 2/10

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