In this HackerRank Insert a node at the head of a linked list problem we need to develop a program in which if we have given a head node pointer of a linked list and the value that we need to add to the list. so we need to create a new node and assign that value to the node and then we need to insert that node at the head of the linked list and then return the reference of a new head node of that linked list.

HackerRank Insert a node at the head of a linked list solution

Problem solution in Python programming.

#!/bin/python3

import math
import os
import random
import re
import sys

class SinglyLinkedListNode:
    def __init__(self, node_data):
        self.data = node_data
        self.next = None

class SinglyLinkedList:
    def __init__(self):
        self.head = None
        self.tail = None

def print_singly_linked_list(node, sep, fptr):
    while node:
        fptr.write(str(node.data))

        node = node.next

        if node:
            fptr.write(sep)


def insertNodeAtHead(llist, data):
    # Write your code here
    node = SinglyLinkedListNode(data)
    if llist:
        node.next = llist
    return node

if __name__ == '__main__':
    fptr = open(os.environ['OUTPUT_PATH'], 'w')

    llist_count = int(input())

    llist = SinglyLinkedList()

    for _ in range(llist_count):
        llist_item = int(input())
        llist_head = insertNodeAtHead(llist.head, llist_item)
        llist.head = llist_head
    
    print_singly_linked_list(llist.head, '\n', fptr)
    fptr.write('\n')
    
    fptr.close()


Problem solution in Java Programming.

import java.io.*;
import java.math.*;
import java.security.*;
import java.text.*;
import java.util.*;
import java.util.concurrent.*;
import java.util.regex.*;

public class Solution {

    static class SinglyLinkedListNode {
        public int data;
        public SinglyLinkedListNode next;

        public SinglyLinkedListNode(int nodeData) {
            this.data = nodeData;
            this.next = null;
        }
    }

    static class SinglyLinkedList {
        public SinglyLinkedListNode head;
        public SinglyLinkedListNode tail;

        public SinglyLinkedList() {
            this.head = null;
            this.tail = null;
        }

      
    }

    public static void printSinglyLinkedList(SinglyLinkedListNode node, String sep, BufferedWriter bufferedWriter) throws IOException {
        while (node != null) {
            bufferedWriter.write(String.valueOf(node.data));

            node = node.next;

            if (node != null) {
                bufferedWriter.write(sep);
            }
        }
    }

   
   static SinglyLinkedListNode insertNodeAtHead(SinglyLinkedListNode llist, int data) {
        SinglyLinkedListNode temp;
        if(llist == null){
            llist = new SinglyLinkedListNode(data);
            return llist;
        }
        temp = new SinglyLinkedListNode(data);
        temp.next = llist;
        llist = temp;
        return llist;
    }

    private static final Scanner scanner = new Scanner(System.in);

    public static void main(String[] args) throws IOException {
        BufferedWriter bufferedWriter = new BufferedWriter(new FileWriter(System.getenv("OUTPUT_PATH")));

        SinglyLinkedList llist = new SinglyLinkedList();

        int llistCount = scanner.nextInt();
        scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])?");

        for (int i = 0; i < llistCount; i++) {
            int llistItem = scanner.nextInt();
            scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])?");
            
          SinglyLinkedListNode llist_head = insertNodeAtHead(llist.head, llistItem);

          llist.head = llist_head;
        }



        printSinglyLinkedList(llist.head, "\n", bufferedWriter);
        bufferedWriter.newLine();

        bufferedWriter.close();

        scanner.close();
    }
}


Problem solution in C++ programming.

#include <bits/stdc++.h>

using namespace std;

class SinglyLinkedListNode {
    public:
        int data;
        SinglyLinkedListNode *next;

        SinglyLinkedListNode(int node_data) {
            this->data = node_data;
            this->next = nullptr;
        }
};

class SinglyLinkedList {
    public:
        SinglyLinkedListNode *head;
        SinglyLinkedListNode *tail;

        SinglyLinkedList() {
            this->head = nullptr;
            this->tail = nullptr;
        }

};

void print_singly_linked_list(SinglyLinkedListNode* node, string sep, ofstream& fout) {
    while (node) {
        fout << node->data;

        node = node->next;

        if (node) {
            fout << sep;
        }
    }
}

void free_singly_linked_list(SinglyLinkedListNode* node) {
    while (node) {
        SinglyLinkedListNode* temp = node;
        node = node->next;

        free(temp);
    }
}

SinglyLinkedListNode* insertNodeAtHead(SinglyLinkedListNode* llist, int data) {
 
 SinglyLinkedListNode *newN = new SinglyLinkedListNode(data);
 newN->next = llist;
 return newN;

}


int main()
{
    ofstream fout(getenv("OUTPUT_PATH"));

    SinglyLinkedList* llist = new SinglyLinkedList();

    int llist_count;
    cin >> llist_count;
    cin.ignore(numeric_limits<streamsize>::max(), '\n');

    for (int i = 0; i < llist_count; i++) {
        int llist_item;
        cin >> llist_item;
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
    
        SinglyLinkedListNode* llist_head = insertNodeAtHead(llist->head, llist_item);
        llist->head = llist_head;
    }

    print_singly_linked_list(llist->head, "\n", fout);
    fout << "\n";

    free_singly_linked_list(llist->head);

    fout.close();

    return 0;
}


Problem solution in C programming.

#include <assert.h>
#include <limits.h>
#include <math.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char* readline();

typedef struct SinglyLinkedListNode SinglyLinkedListNode;
typedef struct SinglyLinkedList SinglyLinkedList;

struct SinglyLinkedListNode {
    int data;
    SinglyLinkedListNode* next;
};

struct SinglyLinkedList {
    SinglyLinkedListNode* head;
    SinglyLinkedListNode* tail;
};

SinglyLinkedListNode* create_singly_linked_list_node(int node_data) {
    SinglyLinkedListNode* node = malloc(sizeof(SinglyLinkedListNode));

    node->data = node_data;
    node->next = NULL;

    return node;
}

void print_singly_linked_list(SinglyLinkedListNode* node, char* sep, FILE* fptr) {
    while (node) {
        fprintf(fptr, "%d", node->data);

        node = node->next;

        if (node) {
            fprintf(fptr, "%s", sep);
        }
    }
}

void free_singly_linked_list(SinglyLinkedListNode* node) {
    while (node) {
        SinglyLinkedListNode* temp = node;
        node = node->next;

        free(temp);
    }
}

// Complete the insertNodeAtHead function below.

/*
 * For your reference:
 *
 * SinglyLinkedListNode {
 *     int data;
 *     SinglyLinkedListNode* next;
 * };
 *
 */
SinglyLinkedListNode* insertNodeAtHead(SinglyLinkedListNode* llist, int data) {
    SinglyLinkedListNode* newnode = create_singly_linked_list_node(data);

    newnode->next = llist;
    return newnode;
}

int main()
{
    FILE* fptr = fopen(getenv("OUTPUT_PATH"), "w");

    SinglyLinkedList* llist = malloc(sizeof(SinglyLinkedList));
    llist->head = NULL;
    llist->tail = NULL;

    char* llist_count_endptr;
    char* llist_count_str = readline();
    int llist_count = strtol(llist_count_str, &llist_count_endptr, 10);

    if (llist_count_endptr == llist_count_str || *llist_count_endptr != '\0') { exit(EXIT_FAILURE); }

    for (int i = 0; i < llist_count; i++) {
        char* llist_item_endptr;
        char* llist_item_str = readline();
        int llist_item = strtol(llist_item_str, &llist_item_endptr, 10);

        if (llist_item_endptr == llist_item_str || *llist_item_endptr != '\0') { exit(EXIT_FAILURE); }

        SinglyLinkedListNode* llist_head = insertNodeAtHead(llist->head, llist_item);
        llist->head = llist_head;
    }



    char *sep = "\n";

    print_singly_linked_list(llist->head, sep, fptr);
    fprintf(fptr, "\n");

    free_singly_linked_list(llist->head);

    fclose(fptr);

    return 0;
}

char* readline() {
    size_t alloc_length = 1024;
    size_t data_length = 0;
    char* data = malloc(alloc_length);

    while (true) {
        char* cursor = data + data_length;
        char* line = fgets(cursor, alloc_length - data_length, stdin);

        if (!line) {
            break;
        }

        data_length += strlen(cursor);

        if (data_length < alloc_length - 1 || data[data_length - 1] == '\n') {
            break;
        }

        alloc_length <<= 1;

        data = realloc(data, alloc_length);

        if (!line) {
            break;
        }
    }

    if (data[data_length - 1] == '\n') {
        data[data_length - 1] = '\0';

        data = realloc(data, data_length);
    } else {
        data = realloc(data, data_length + 1);

        data[data_length] = '\0';
    }

    return data;
}


Problem solution in JavaScript programming.

'use strict';

const fs = require('fs');

process.stdin.resume();
process.stdin.setEncoding('utf-8');

let inputString = '';
let currentLine = 0;

process.stdin.on('data', inputStdin => {
    inputString += inputStdin;
});

process.stdin.on('end', function() {
    inputString = inputString.replace(/\s*$/, '')
        .split('\n')
        .map(str => str.replace(/\s*$/, ''));

    main();
});

function readLine() {
    return inputString[currentLine++];
}

const SinglyLinkedListNode = class {
    constructor(nodeData) {
        this.data = nodeData;
        this.next = null;
    }
};

const SinglyLinkedList = class {
    constructor() {
        this.head = null;
        this.tail = null;
    }

};

function printSinglyLinkedList(node, sep, ws) {
    while (node != null) {
        ws.write(String(node.data));

        node = node.next;

        if (node != null) {
            ws.write(sep);
        }
    }
}

// Complete the insertNodeAtHead function below.

/*
 * For your reference:
 *
 * SinglyLinkedListNode {
 *     int data;
 *     SinglyLinkedListNode next;
 * }
 *
 */
function insertNodeAtHead(head, data) {
    if(head == null){
        return new SinglyLinkedListNode(data);
    }
    var node = new SinglyLinkedListNode(data);
    node.next = head;
    return node;
}


function main() {
    const ws = fs.createWriteStream(process.env.OUTPUT_PATH);

    const llistCount = parseInt(readLine(), 10);

    let llist = new SinglyLinkedList();

    for (let i = 0; i < llistCount; i++) {
        const llistItem = parseInt(readLine(), 10);
        const llist_head = insertNodeAtHead(llist.head, llistItem);
        llist.head = llist_head;
    }



    printSinglyLinkedList(llist.head, '\n', ws);
    ws.write('\n');

    ws.end();
}