Reverse first K elements of given linked list - GeeksforGeeks

22 Jun, 2021
Given a pointer to the head node of a linked list and a number K, the task is to reverse the first K nodes of the linked list. We need to reverse the list by changing links between nodes. check also Reversal of a linked list
Examples: 
Input : 1->2->3->4->5->6->7->8->9->10->NULL
        k = 3
Output :3->2->1->4->5->6->7->8->9->10->NULL

Input :10->18->20->25->35->NULL
       k = 2
Output :18->10->20->25->35->NULL 
Explanation of the method: suppose linked list is 1->2->3->4->5->NULL and k=31) Traverse the linked list till K-th point. 2) Break the linked list in to two parts from k-th point. After partition linked list will look like 1->2->3->NULL & 4->5->NULL 3) Reverse first part of the linked list leave second part as it is 3->2->1->NULL and 4->5->NULL 4) Join both the parts of the linked list, we get 3->2->1->4->5->NULLA pictorial representation of how the algorithm works 
C++
Java
Python
C#
Javascript
// C++ program for reversal of first k elements// of given linked list#include <bits/stdc++.h>using namespace std; /* Link list node */struct Node {    int data;    struct Node* next;}; /* Function to reverse first k elements of linked list */static void reverseKNodes(struct Node** head_ref, int k){    // traverse the linked list until break    // point not meet    struct Node* temp = *head_ref;    int count = 1;    while (count < k) {        temp = temp->next;        count++;    }     // backup the joint point    struct Node* joint_point = temp->next;    temp->next = NULL; // break the list     // reverse the list till break point    struct Node* prev = NULL;    struct Node* current = *head_ref;    struct Node* next;    while (current != NULL) {        next = current->next;        current->next = prev;        prev = current;        current = next;    }     // join both parts of the linked list    // traverse the list until NULL is not    // found    *head_ref = prev;    current = *head_ref;    while (current->next != NULL)        current = current->next;     // joint both part of the list    current->next = joint_point;} /* Function to push a node */void push(struct Node** head_ref, int new_data){    struct Node* new_node =          (struct Node*)malloc(sizeof(struct Node));    new_node->data = new_data;    new_node->next = (*head_ref);    (*head_ref) = new_node;} /* Function to print linked list */void printList(struct Node* head){    struct Node* temp = head;    while (temp != NULL) {        printf("%d ", temp->data);        temp = temp->next;    }} /* Driver program to test above function*/int main(){    // Create a linked list 1->2->3->4->5    struct Node* head = NULL;    push(&head, 5);    push(&head, 4);    push(&head, 3);    push(&head, 2);    push(&head, 1);     // k should be less than the    // numbers of nodes    int k = 3;     cout << "\nGiven list\n";    printList(head);     reverseKNodes(&head, k);     cout << "\nModified list\n";    printList(head);     return 0;}
// Java program for reversal of first k elements// of given linked listclass Sol{     // Link list nodestatic class Node{    int data;    Node next;}; // Function to reverse first k elements of linked liststatic Node reverseKNodes( Node head_ref, int k){    // traverse the linked list until break    // point not meet    Node temp = head_ref;    int count = 1;    while (count < k)    {        temp = temp.next;        count++;    }     // backup the joint point    Node joint_point = temp.next;    temp.next = null; // break the list     // reverse the list till break point    Node prev = null;    Node current = head_ref;    Node next;    while (current != null)    {        next = current.next;        current.next = prev;        prev = current;        current = next;    }     // join both parts of the linked list    // traverse the list until null is not    // found    head_ref = prev;    current = head_ref;    while (current.next != null)        current = current.next;     // joint both part of the list    current.next = joint_point;    return head_ref;} // Function to push a nodestatic Node push( Node head_ref, int new_data){    Node new_node = new Node();    new_node.data = new_data;    new_node.next = (head_ref);    (head_ref) = new_node;    return head_ref;} // Function to print linked liststatic void printList( Node head){    Node temp = head;    while (temp != null)    {        System.out.printf("%d ", temp.data);        temp = temp.next;    }} // Driver program to test above functionpublic static void main(String args[]){    // Create a linked list 1.2.3.4.5    Node head = null;    head = push(head, 5);    head = push(head, 4);    head = push(head, 3);    head = push(head, 2);    head = push(head, 1);     // k should be less than the    // numbers of nodes    int k = 3;     System.out.print("\nGiven list\n");    printList(head);     head = reverseKNodes(head, k);     System.out.print("\nModified list\n");    printList(head);}} // This code is contributed by Arnab Kundu
# Python program for reversal of first k elements# of given linked list # Node of a linked listclass Node:    def __init__(self, next = None, data = None):        self.next = next        self.data = data # Function to reverse first k elements of linked listdef reverseKNodes(head_ref, k) :     # traverse the linked list until break    # point not meet    temp = head_ref    count = 1    while (count < k):             temp = temp.next        count = count + 1         # backup the joint point    joint_point = temp.next    temp.next = None # break the list     # reverse the list till break point    prev = None    current = head_ref    next = None    while (current != None):             next = current.next        current.next = prev        prev = current        current = next         # join both parts of the linked list    # traverse the list until None is not    # found    head_ref = prev    current = head_ref    while (current.next != None):        current = current.next     # joint both part of the list    current.next = joint_point    return head_ref # Function to push a nodedef push(head_ref, new_data) :     new_node = Node()    new_node.data = new_data    new_node.next = (head_ref)    (head_ref) = new_node    return head_ref # Function to print linked listdef printList( head) :     temp = head    while (temp != None):             print(temp.data, end = " ")        temp = temp.next     # Driver program to test above function # Create a linked list 1.2.3.4.5head = Nonehead = push(head, 5)head = push(head, 4)head = push(head, 3)head = push(head, 2)head = push(head, 1) # k should be less than the# numbers of nodesk = 3 print("\nGiven list")printList(head) head = reverseKNodes(head, k) print("\nModified list")printList(head) # This code is contributed by Arnab Kundu
// C# program for reversal of first k elements// of given linked listusing System;     class GFG{     // Link list nodepublic class Node{    public int data;    public Node next;}; // Function to reverse first k elements of linked liststatic Node reverseKNodes(Node head_ref, int k){         // traverse the linked list until break    // point not meet    Node temp = head_ref;    int count = 1;    while (count < k)    {        temp = temp.next;        count++;    }     // backup the joint point    Node joint_point = temp.next;    temp.next = null; // break the list     // reverse the list till break point    Node prev = null;    Node current = head_ref;    Node next;    while (current != null)    {        next = current.next;        current.next = prev;        prev = current;        current = next;    }     // join both parts of the linked list    // traverse the list until null is not    // found    head_ref = prev;    current = head_ref;    while (current.next != null)        current = current.next;     // joint both part of the list    current.next = joint_point;    return head_ref;} // Function to push a nodestatic Node push( Node head_ref, int new_data){    Node new_node = new Node();    new_node.data = new_data;    new_node.next = (head_ref);    (head_ref) = new_node;    return head_ref;} // Function to print linked liststatic void printList( Node head){    Node temp = head;    while (temp != null)    {        Console.Write("{0} ", temp.data);        temp = temp.next;    }} // Driver Codepublic static void Main(String []args){    // Create a linked list 1.2.3.4.5    Node head = null;    head = push(head, 5);    head = push(head, 4);    head = push(head, 3);    head = push(head, 2);    head = push(head, 1);     // k should be less than the    // numbers of nodes    int k = 3;     Console.Write("Given list\n");    printList(head);     head = reverseKNodes(head, k);     Console.Write("\nModified list\n");    printList(head);}} // This code is contributed by Princi Singh
<script> // Javascript program for reversal of first k elements// of given linked list     // Link list nodeclass Node{    constructor()    {        this.data = 0;        this.next = null;    }}; // Function to reverse first k elements of linked listfunction reverseKNodes(head_ref, k){         // traverse the linked list until break    // point not meet    var temp = head_ref;    var count = 1;    while (count < k)    {        temp = temp.next;        count++;    }     // backup the joint point    var joint_point = temp.next;    temp.next = null; // break the list     // reverse the list till break point    var prev = null;    var current = head_ref;    var next;    while (current != null)    {        next = current.next;        current.next = prev;        prev = current;        current = next;    }     // join both parts of the linked list    // traverse the list until null is not    // found    head_ref = prev;    current = head_ref;    while (current.next != null)        current = current.next;     // joint both part of the list    current.next = joint_point;    return head_ref;} // Function to push a nodefunction push( head_ref, new_data){    var new_node = new Node();    new_node.data = new_data;    new_node.next = (head_ref);    (head_ref) = new_node;    return head_ref;} // Function to print linked listfunction printList( head){    var temp = head;    while (temp != null)    {        document.write(temp.data+ " ");        temp = temp.next;    }} // Driver Code// Create a linked list 1.2.3.4.5var head = null;head = push(head, 5);head = push(head, 4);head = push(head, 3);head = push(head, 2);head = push(head, 1);// k should be less than the// numbers of nodesvar k = 3;document.write("Given list<br>");printList(head);head = reverseKNodes(head, k);document.write("<br>Modified list<br>");printList(head); </script>
Output:  
Given list
1 2 3 4 5 
Modified list
3 2 1 4 5 
Time Complexity : O(n)  
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