HackerRank•Jun 10, 2025

Insert a node at the head of a linked list

Hazrat Ali

HackerRank

Given a pointer to the head of a linked list, insert a new node before the head. The  value in the new node should point to  and the  value should be replaced with a given value. Return a reference to the new head of the list. The head pointer given may be null meaning that the initial list is empty.

Function Description

Complete the function  with the following parameter(s):

  • a reference to the head of a list
  • the value to insert in the  field of the new node

Input Format

The first line contains an integer , the number of elements to be inserted at the head of the list.
The next  lines contain an integer each, the elements to be inserted, one per function call.

Constraints

Sample Input

STDIN   Function
-----   --------
5       n = 5
383     data items to insert 383 ... 321
484
392
975
321

Sample Output

321
975
392
484
383


Solution
#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);
    }
}

// Complete the insertNodeAtHead function below.

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

    if (llist == nullptr) return node;

    node->next = llist;
    llist = node;

    return llist;
}
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;
}





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