Implementation of Priority Queue using Linked List in C

Implementation of Priority Queue using Linked List

In this post, we will learn to implement priority queues using Linked Lists.

It has the following methods –

  • Push()
  • Pop()
  • Peek()

Do not worry it’s not enqueue/dequeue as pushing and popping happens in accordance with priority not from the end/front as in the case of queue

prioritize

Operations

  • Push: Pushing/inserting a new item in the priority queue at a position in accordance with its priority
  • Pop: Popping/removing a new item in the priority queue in accordance with its priority
  • Peek: Displaying the topmost priority item of the queue

Syntax for Priority Queue as Linked List

struct Node
{
	int data;
	int priority; // lower value means higher priority
	struct Node* next;
};

Advantage of implementing Priority Queue as Linked List over Array

  • Implementation of priority Queue data by using a linked list is more efficient as compared to arrays.
  • The linked list provides you with the facility of Dynamic memory allocation.
  • The memory is not wasted as memory, not in use can be freed, using free(); method
Implementation of Priority Queue using Linked List in C

C Program to Implement Priority Queue using Linked list

Code in C

#include<stdio.h>
#include<stdlib.h>

struct Node
{
	int data;
	int priority; // lower value means higher priority
	struct Node* next;
};

// Function to Create A New Node
Node* create(int data, int priorityVal)
{
    Node* temp = (Node*) malloc(sizeof(Node));
    temp->data = data;
    temp->priority = priorityVal;
    temp->next = NULL;
 
    return temp;
}

// Return the value at head
int peek(Node** head)
{
    return (*head)->data;
}

// Function to push in accodance with priority
void push(Node** head, int data, int priorityVal)
{
    Node* curr = (*head);
 
    // Create new Node
    Node* temp = create(data, priorityVal);
 
    // If incoming node has lower priority value
    // than the current head. This incoming node
    // is inserted before head
    // Note: Lower priority value means higher priority
    if ((*head)->priority > priorityVal) {
 
        // new node inserted before head
        temp->next = *head;
        (*head) = temp;
    }
    else {
 
        // else we traverse the list to find 
        // correct position to insert the incoming node
        while (curr->next != NULL &&
            curr->next->priority < priorityVal) { curr = curr->next;
        }
 
        // incoming node inserted here
        // either at req. position or end of the list
        temp->next = curr->next;
        curr->next = temp;
    }
}

// Here we remove the element with highest priotity 
// highest priority will be at the head itself
void pop(Node** head)
{
    Node* temp = *head;
    (*head) = (*head)->next;
    printf("(%d, priority: %d) popped\n",temp->data,temp->priority);
    free(temp);
}

// Function to check is list is empty
int isEmpty(Node** node)
{
    return (*node) == NULL;
}

void display(struct Node* node){
    printf("Priority Queue: ");
    // as linked list will end when Node is Null
    while(node!=NULL)
    {
        printf("%d ",node->data);
        node = node->next;
    }
    printf("\n");
}
int main()
{
    Node* pq = create(10, 1);
	push(&pq, 30, 3);
	push(&pq, 20, 2);
	push(&pq, 40, 4);
	
	display(pq);
	
	pop(&pq);
	pop(&pq);
	
	display(pq);
	
	push(&pq, 15, 2);
	// if two items have same priority then
	// they are served in their order of entry
	push(&pq, 20, 2);
	push(&pq, 50, 5);
	push(&pq, 5, 1);
	
	display(pq);

    return 0;
}

Output

Priority Queue: 10 20 30 40 
(10, priority: 1) popped
(20, priority: 2) popped
Priority Queue: 30 40 
Priority Queue: 5 15 20 30 40 50

Queue using Array

Here we have already learned implementation of Priority Queue using Linked List in C , click the button below to learn the implementation of queue using array