Sum of all odd frequency nodes of the linked list

// C Program
// Sum of all odd frequency nodes of the linked list
#include <stdio.h>
 //For malloc function
#include <stdlib.h>

//Linked List Node
struct Node
{
	int data;
	struct Node *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 
void insert(struct Node **head, int data)
{
	struct Node *node = create_node(data);
	if ( *head == NULL)
	{
		*head = node;
	}
	else
	{
		struct Node *temp = *head;
		//Find last node
		while (temp->next != NULL)
		{
			temp = temp->next;
		}
		//Add node at last possition
		temp->next = node;
	}
}
//Display linked list element
void display(struct Node *head)
{
	if (head == NULL)
	{
		printf("\nEmpty linked list\n");
		return;
	}
	struct Node *temp = head;
	printf("\n Linked List : ");
	while (temp != NULL)
	{
		printf("  %d", temp->data);
		//visit to next node
		temp = temp->next;
	}
	printf("\n");
}
// This function are takes O(N^2) time to combine similar elements
void group_element(struct Node *head)
{
	//Define some auxiliary variables
	struct Node *outer = head;
	struct Node *inner = NULL;
	struct Node *auxiliary = NULL;
	int temp = 0;
	while (outer != NULL)
	{
		auxiliary = outer->next;
		inner = auxiliary;
		while (inner != NULL && auxiliary != NULL)
		{
			if (auxiliary->data == outer->data)
			{
				//visit to next node
				auxiliary = auxiliary->next;
				if (auxiliary != NULL)
				{
					inner = auxiliary->next;
				}
				else
				{
					inner = auxiliary;
				}
			}
			else if (outer->data == inner->data)
			{
				//Swap node value
				temp = auxiliary->data;
				auxiliary->data = inner->data;
				inner->data = temp;
				//visit to next upcomming nodes
				auxiliary = auxiliary->next;
				inner = inner->next;
			}
			else
			{
				inner = inner->next;
			}
		}
		outer = auxiliary;
	}
}
//Sum the all elements which are occurs in odd number of team
void sum_odd_frequency_nodes(struct Node *head)
{
	if (head == NULL)
	{
		//When linked list is empty
		return;
	}
	//Grouping of similar nodes elements
	group_element(head);
	//Use to iterating linked list nodes
	struct Node *outer = head;
	struct Node *inner = NULL;
	//Define resultant variables
	int counter = 0;
	int result = 0;
	// This loop takes O(N) time to find the frequency of all nodes
	while (outer != NULL)
	{
		//Set node counter
		counter = 1;
		//start to next node
		inner = outer->next;
		//Count repeated nodes
		while (inner != NULL && inner->data == outer->data)
		{
			counter++;
			inner = inner->next;
		}
		if ((counter % 2) != 0)
		{
			if (outer != head)
			{
				printf(" +");
			}
			printf(" (%dx%d)", outer->data, counter);
			//When odd frequency node get
			result += (outer->data) *counter;
		}
		//Get next frequency node in list
		outer = inner;
	}
	printf("\n Odd frequency node sum is : %d\n", result);
}
int main()
{
	struct Node *head = NULL;
	//Create linked list
	insert( &head, 4);
	insert( &head, 7);
	insert( &head, 2);
	insert( &head, 3);
	insert( &head, 4);
	insert( &head, 3);
	insert( &head, 4);
	insert( &head, 3);
	insert( &head, 2);
	insert( &head, 4);
	insert( &head, 4);
	//Display of linked list nodes
	display(head);
	sum_odd_frequency_nodes(head);
	return 0;
}

Output

 Linked List :   4  7  2  3  4  3  4  3  2  4  4
 (4x5) + (3x3) + (7x1)
 Odd frequency node sum is : 36