Append the last M nodes to the beginning of the given linked list

Difficulty Level : Medium
Last Updated : 18 Jun, 2021

Given a linked list and an integer M, the task is to append the last M nodes of the linked list to the front.
Examples:

Input: List = 4 -> 5 -> 6 -> 1 -> 2 -> 3 -> NULL, M = 3
Output: 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> NULL
Input: List = 8 -> 7 -> 0 -> 4 -> 1 -> NULL, M = 2
Output: 4 -> 1 -> 8 -> 7 -> 0 -> NULL

Approach: Find the first node of the last M nodes in the list, this node will be the new head node so make the next pointer of the previous node as NULL and point the last node of the original list to the head of the original list. Finally, print the updated list.
Below is the implementation of the above approach:

C++

// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
 
// Class for a node of
// the linked list
struct Node
{
 
    // Data and the pointer
    // to the next node
    int data;
    Node *next;
    Node(int data)
    {
        this->data = data;
        this->next = NULL;
    }
};
 
// Function to print the linked list
void printList(Node* node)
{
  while (node != NULL)
  {
    cout << (node->data) << " -> ";
    node = node->next;
  }
  cout << "NULL";
}
 
// Recursive function to return the
// count of nodes in the linked list
int cntNodes(Node* node)
{
  if (node == NULL)
    return 0;
 
  return (1 + cntNodes(node->next));
}
 
// Function to update and print
// the updated list nodes
void updateList(Node* head, int m)
{
 
  // Total nodes in the list
  int cnt = cntNodes(head);
 
  if (cnt != m && m < cnt)
  {
 
    // Count of nodes to be skipped
    // from the beginning
    int skip = cnt - m;
    Node *prev = NULL;
    Node *curr = head;
 
    // Skip the nodes
    while (skip > 0)
    {
      prev = curr;
      curr = curr->next;
      skip--;
    }
 
    // Change the pointers
    prev->next = NULL;
    Node *tempHead = head;
    head = curr;
 
    // Find the last node
    while (curr->next != NULL)
      curr = curr->next;
 
    // Connect it to the head
    // of the sub list
    curr->next = tempHead;
  }
 
  // Print the updated list
  printList(head);
}
 
// Driver code
int main()
{
 
  // Create the list
  Node* head = new Node(4);
  head->next = new Node(5);
  head->next->next = new Node(6);
  head->next->next->next = new Node(1);
  head->next->next->next->next = new Node(2);
  head->next->next->next->next->next = new Node(3);
  int m = 3;
  updateList(head, m);
  return 0;
}
 
// This code is contributed by rutvik_56

Java

// Java implementation of the approach
class GFG {
 
    // Class for a node of
    // the linked list
    static class Node {
 
        // Data and the pointer
        // to the next node
        int data;
        Node next;
 
        Node(int data)
        {
            this.data = data;
            this.next = null;
        }
    }
 
    // Function to print the linked list
    static void printList(Node node)
    {
        while (node != null) {
            System.out.print(node.data + " -> ");
            node = node.next;
        }
        System.out.print("NULL");
    }
 
    // Recursive function to return the
    // count of nodes in the linked list
    static int cntNodes(Node node)
    {
        if (node == null)
            return 0;
 
        return (1 + cntNodes(node.next));
    }
 
    // Function to update and print
    // the updated list nodes
    static void updateList(Node head, int m)
    {
 
        // Total nodes in the list
        int cnt = cntNodes(head);
 
        if (cnt != m && m < cnt) {
 
            // Count of nodes to be skipped
            // from the beginning
            int skip = cnt - m;
            Node prev = null;
            Node curr = head;
 
            // Skip the nodes
            while (skip > 0) {
                prev = curr;
                curr = curr.next;
                skip--;
            }
 
            // Change the pointers
            prev.next = null;
            Node tempHead = head;
            head = curr;
 
            // Find the last node
            while (curr.next != null)
                curr = curr.next;
 
            // Connect it to the head
            // of the sub list
            curr.next = tempHead;
        }
 
        // Print the updated list
        printList(head);
    }
 
    // Driver code
    public static void main(String[] args)
    {
 
        // Create the list
        Node head = new Node(4);
        head.next = new Node(5);
        head.next.next = new Node(6);
        head.next.next.next = new Node(1);
        head.next.next.next.next = new Node(2);
        head.next.next.next.next.next = new Node(3);
 
        int m = 3;
 
        updateList(head, m);
    }
}

Python3

# Python3 implementation of the approach
 
# Class for a node of
# the linked list
class newNode:
     
    # Constructor to initialize the node object
    def __init__(self, data):
        self.data = data
        self.next = None
 
# Function to print the linked list
def printList(node):
     
    while (node != None):
        print(node.data,"->", end=" ")
        node = node.next
    print("NULL")
 
# Recursive function to return the
# count of nodes in the linked list
def cntNodes(node):
    if (node == None):
        return 0
         
    return (1 + cntNodes(node.next))
     
# Function to update and print
# the updated list nodes
def updateList(head, m):
     
    # Total nodes in the list
    cnt = cntNodes(head)
     
    if (cnt != m and m < cnt):
         
        # Count of nodes to be skipped
        # from the beginning
        skip = cnt - m
        prev = None
         
        curr = head
         
        # Skip the nodes
        while (skip > 0):
            prev = curr
            curr = curr.next
            skip-=1
             
        # Change the pointers
        prev.next = None
        tempHead = head
        head = curr
         
        # Find the last node
        while (curr.next != None):
            curr = curr.next
             
        # Connect it to the head
        # of the sub list
        curr.next = tempHead
     
    # Print the updated list
    printList(head)
 
# Driver code
 
# Create the list
head = newNode(4)
head.next = newNode(5)
head.next.next = newNode(6)
head.next.next.next = newNode(1)
head.next.next.next.next = newNode(2)
head.next.next.next.next.next = newNode(3)
 
m = 3
 
updateList(head, m)
 
# This code is contributed by shubhamsingh10

C#

// C# implementation of the approach
using System;
 
class GFG
{
 
    // Class for a node of
    // the linked list
    class Node
    {
 
        // Data and the pointer
        // to the next node
        public int data;
        public Node next;
 
        public Node(int data)
        {
            this.data = data;
            this.next = null;
        }
    }
 
    // Function to print the linked list
    static void printList(Node node)
    {
        while (node != null)
        {
            Console.Write(node.data + " -> ");
            node = node.next;
        }
        Console.Write("NULL");
    }
 
    // Recursive function to return the
    // count of nodes in the linked list
    static int cntNodes(Node node)
    {
        if (node == null)
            return 0;
 
        return (1 + cntNodes(node.next));
    }
 
    // Function to update and print
    // the updated list nodes
    static void updateList(Node head, int m)
    {
 
        // Total nodes in the list
        int cnt = cntNodes(head);
 
        if (cnt != m && m < cnt)
        {
 
            // Count of nodes to be skipped
            // from the beginning
            int skip = cnt - m;
            Node prev = null;
            Node curr = head;
 
            // Skip the nodes
            while (skip > 0)
            {
                prev = curr;
                curr = curr.next;
                skip--;
            }
 
            // Change the pointers
            prev.next = null;
            Node tempHead = head;
            head = curr;
 
            // Find the last node
            while (curr.next != null)
                curr = curr.next;
 
            // Connect it to the head
            // of the sub list
            curr.next = tempHead;
        }
 
        // Print the updated list
        printList(head);
    }
 
    // Driver code
    public static void Main(String[] args)
    {
 
        // Create the list
        Node head = new Node(4);
        head.next = new Node(5);
        head.next.next = new Node(6);
        head.next.next.next = new Node(1);
        head.next.next.next.next = new Node(2);
        head.next.next.next.next.next = new Node(3);
 
        int m = 3;
 
        updateList(head, m);
    }
}
 
// This code is contributed by PrinciRaj1992

Javascript

<script>
      // JavaScript implementation of the approach
      // Class for a node of
      // the linked list
      class Node {
        // Data and the pointer
        // to the next node
 
        constructor(data) {
          this.data = data;
          this.next = null;
        }
      }
 
      // Function to print the linked list
      function printList(node) {
        while (node != null) {
          document.write(node.data + " -> ");
          node = node.next;
        }
        document.write("NULL");
      }
 
      // Recursive function to return the
      // count of nodes in the linked list
      function cntNodes(node) {
        if (node == null) return 0;
 
        return 1 + cntNodes(node.next);
      }
 
      // Function to update and print
      // the updated list nodes
      function updateList(head, m) {
        // Total nodes in the list
        var cnt = cntNodes(head);
 
        if (cnt != m && m < cnt) {
          // Count of nodes to be skipped
          // from the beginning
          var skip = cnt - m;
          var prev = null;
          var curr = head;
 
          // Skip the nodes
          while (skip > 0) {
            prev = curr;
            curr = curr.next;
            skip--;
          }
 
          // Change the pointers
          prev.next = null;
          var tempHead = head;
          head = curr;
 
          // Find the last node
          while (curr.next != null) curr = curr.next;
 
          // Connect it to the head
          // of the sub list
          curr.next = tempHead;
        }
 
        // Print the updated list
        printList(head);
      }
 
      // Driver code
      // Create the list
      var head = new Node(4);
      head.next = new Node(5);
      head.next.next = new Node(6);
      head.next.next.next = new Node(1);
      head.next.next.next.next = new Node(2);
      head.next.next.next.next.next = new Node(3);
 
      var m = 3;
 
      updateList(head, m);
       
      // This code is contributed by rdtank.
    </script>

Output:

1 -> 2 -> 3 -> 4 -> 5 -> 6 -> NULL

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