Sort the linked list elements by node frequency

// C Program 
// Sort the linked list elements by node frequency
#include <stdio.h>

#include <stdlib.h> //for malloc function

//Linked List Node
struct Node
{
	int data;
	struct Node *next;
};
//Linked List Node
struct Frequency
{
	int key;
	int counter;
	struct Frequency *next;
};
//Create a node of linked list
struct Node *create_node(int data)
{
	//Create dynamic node
	struct Node *node = (struct Node *) malloc(sizeof(struct Node));
	if (node == NULL)
	{
		printf("Memory overflow\n");
	}
	else
	{
		//Set initial node value
		node->data = data;
		node->next = NULL;
	}
	return node;
}
//Add new node at end of linked list 
struct Node *add_node(struct Node *tail, int data)
{
	struct Node *node = create_node(data);
	if (tail != NULL)
	{
		tail->next = node;
	}
	return node;
}
struct Frequency *add_frequency(struct Frequency *collection, int key, int counter)
{
	//Create dynamic node
	struct Frequency *element = (struct Frequency *) malloc(sizeof(struct Frequency));
	if (element == NULL)
	{
		printf("Memory overflow\n");
	}
	else
	{
		// Add the value of new element 
		element->key = key;
		element->counter = counter;
		element->next = NULL;
		if (collection == NULL)
		{
			return element;
		}
		else if (collection->counter >= counter)
		{
			//When need to add node at beginning
			element->next = collection;
			return element;
		}
		else
		{
			//sorted add new node
			struct Frequency *temp = collection;
			while (temp != NULL && temp->next != NULL && temp->next->counter < counter)
			{
				//visit to next node
				temp = temp->next;
			}
			element->next = temp->next;
			//add element into given collection
			temp->next = element;
		}
	}
	return collection;
}
//Display linked list element
void display(struct Node *head)
{
	if (head == NULL)
	{
		printf("\n Empty linked list\n");
		return;
	}
	struct Node *temp = head;
	//iterating linked list elements
	while (temp != NULL)
	{
		if (temp != head)
		{
			printf(" →");
		}
		printf(" %d", temp->data);
		//visit to next node
		temp = temp->next;
	}
	printf(" → NULL\n");
}
struct Node *count_and_delete(struct Node *first, int *counter, int key)
{
	//Define some auxiliary variables
	struct Node *current = first;
	struct Node *front = first;
	struct Node *auxiliary = NULL;
	struct Node *back = NULL;
	first = NULL;
	//iterating linked list elements
	while (current != NULL)
	{
		if (current->data == key)
		{
			//When get delete node node
			auxiliary = current;
			//node counter
			*counter = ( *counter) + 1;
		}
		else
		{
			back = current;
		}
		//visit to next node
		current = current->next;
		if (auxiliary != NULL)
		{
			//When Deleted node exists
			if (back == NULL)
			{
				// When front node is delete
				// head visit to next node
				front = current;
			}
			else
			{
				// Deleted node are exist in intermediate or last position of linked list
				// Before delete node, there left node is connecting to next upcoming node
				back->next = current;
			}
			//unlink deleted node 
			auxiliary->next = NULL;
			//free node
			free(auxiliary);
			auxiliary = NULL;
		}
	}
	return front;
}
void sort_by_frequency(struct Node **head, struct Node **tail)
{
	if (head == NULL)
	{
		printf("\n Empty linked List");
	}
	else
	{
		struct Frequency *collection = NULL;
		struct Frequency *temp = NULL;
		int counter = 0;
		int key = 0;
		struct Node *current = *head;
		//set last node to null
		*tail = NULL;
		//Count node frequency and arrange in sorted order
		while (current != NULL)
		{
			key = current->data;*head = count_and_delete( *head, &counter, key);
			collection = add_frequency(collection, key, counter);
			current = *head;
			counter = 0;
		}
		while (collection != NULL)
		{
			counter = collection->counter;
			key = collection->key;
			while (counter > 0)
			{
				*tail = add_node( *tail, key);
				if ( *head == NULL)
				{
					*head = *tail;
				}
				counter--;
			}
			temp = collection;
			//visit to next node
			collection = collection->next;
			//Free node
			temp->next = NULL;
			free(temp);
			temp = NULL;
		}
	}
}
int main()
{
	struct Node *head = NULL;
	struct Node *tail = NULL;
	// Add linked list node
	head = tail = add_node(tail, 2);
	tail = add_node(tail, 6);
	tail = add_node(tail, 7);
	tail = add_node(tail, 6);
	tail = add_node(tail, 3);
	tail = add_node(tail, 6);
	tail = add_node(tail, 2);
	tail = add_node(tail, 3);
	tail = add_node(tail, 1);
	tail = add_node(tail, 7);
	tail = add_node(tail, 3);
	tail = add_node(tail, 6);
	tail = add_node(tail, 7);
	tail = add_node(tail, 1);
	tail = add_node(tail, 5);
	tail = add_node(tail, 6);
	tail = add_node(tail, 9);
	tail = add_node(tail, 3);
	printf("Before sort by frequency\n");
	// 2 → 6 → 7 → 6 → 3 → 6 → 2 → 3 → 1 → 7 → 3 → 6 → 7 → 1 → 5 → 6 → 9 → 3 → NULL
	display(head);
	sort_by_frequency( &head, &tail);
	printf("Before sort by frequency\n");
	//9 → 5 → 1 → 1 → 2 → 2 → 7 → 7 → 7 → 3 → 3 → 3 → 3 → 6 → 6 → 6 → 6 → 6 → NULL
	display(head);
	return 0;
}

Output

Before sort by frequency
 2 → 6 → 7 → 6 → 3 → 6 → 2 → 3 → 1 → 7 → 3 → 6 → 7 → 1 → 5 → 6 → 9 → 3 → NULL
Before sort by frequency
 9 → 5 → 1 → 1 → 2 → 2 → 7 → 7 → 7 → 3 → 3 → 3 → 3 → 6 → 6 → 6 → 6 → 6 → NULL