Priority Queue: Priority queue is a type of queue where each element has a priority value and the deletion of the elements is depended upon the priority value. In case of max-priority queue, the element will be deleted first which has the largest priority value and in case of min-priority queue the element will be deleted first which has the minimum priority value.

// program to implement priority queue

# include <stdio.h> 
# include <stdlib.h> 
struct node 
{ 
int data; 
int priority; 
struct node *link; 
}; 
void insert(struct node **front, struct node **rear, int value, int 
priority) 
{ 
struct node *temp,*temp1; 
temp=(struct node *)malloc(sizeof(struct node)); 
/* creates new node using data value 
passed as parameter */ 
if(temp==NULL) 
{ 
printf("No Memory available Error\n"); 
exit(0); 
} 
temp->data = value; 
temp->priority = priority; 
temp->link=NULL; 
if(*rear == NULL) /* This is the first node */ 
{ 
*rear = temp; 
*front = *rear; 
} 
else 
{ 
if((*front)->priority < priority) 
/* the element to be inserted has 
highest priority hence should 
be the first element*/ 
{
temp->link = *front; 
*front = temp; 
} 
else 
if( (*rear)->priority > priority) 
/* the element to be inserted has 
lowest priority hence should 
be the last element*/ 
{ 
(*rear)->link = temp; 
*rear = temp; 
} 
else 
{ 
temp1 = *front; 
while((temp1->link)->priority >= priority) 
/* find the position and insert the new element */ 
temp1=temp1->link; 
temp->link = temp1->link; 
temp1->link = temp; 
} 
} 
void delete(struct node **front, struct node **rear, int *value, int 
*priority) 
{ 
struct node *temp; 
if((*front == *rear) && (*rear == NULL)) 
{ 
printf(" The queue is empty cannot delete Error\n"); 
exit(0); 
} 
*value = (*front)->data; 
*priority = (*front)->priority; 
temp = *front; 
*front = (*front)->link; 
if(*rear == temp) 
*rear = (*rear)->link; 
free(temp); 
} 
void main() 
{ 
struct node *front=NULL,*rear = NULL; 
int n,value, priority; 
do 
{ 
do 
{ 
printf("Enter the element to be inserted and its priority\n"); 
scanf("%d %d",&value,&priority); 
insert(&front,&rear,value,priority); 
printf("Enter 1 to continue\n"); 
scanf("%d",&n); 
} while(n == 1); 
printf("Enter 1 to delete an element\n"); 
scanf("%d",&n); 
while( n == 1) 
{ 
delete(&front,&rear,&value,&priority); 
printf("The value deleted is %d\ and its priority is %d \n", 
value,priority); 
printf("Enter 1 to delete an element\n"); 
scanf("%d",&n); 
} 
printf("Enter 1 to delete an element\n"); 
scanf("%d",&n); 
} while( n == 1)