nsert a node at the head of a linked list C++ hackerrank

#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* head, int data) {
        if(head==NULL)
        {
            head=(SinglyLinkedListNode*)malloc(sizeof(SinglyLinkedListNode));
            head->data=data;
            head->next=NULL;
        }
        else
        {
            SinglyLinkedListNode* p;
            p=(SinglyLinkedListNode*)malloc(sizeof(SinglyLinkedListNode));
            p->data=data;
            p->next=head;
            head=p;    
        }
        return head;

}

int main()