Implement Priority Queue using Linked Lists.
- push(): This function is used to insert a new data into the queue.
- pop(): This function removes the element with the highest priority from the queue.
- peek() / top(): This function is used to get the highest priority element in the queue without removing it from the queue.
Priority Queues can be implemented using common data structures like arrays, linked-lists, heaps and binary trees.
Prerequisites :
Linked Lists, Priority Queues
The list is so created so that the highest priority element is always at the head of the list. The list is arranged in descending order of elements based on their priority. This allow us to remove the highest priority element in O(1) time. To insert an element we must traverse the list and find the proper position to insert the node so that the overall order of the priority queue is maintained. This makes the push() operation takes O(N) time. The pop() and peek() operations are performed in constant time.
Algorithm :
PUSH(HEAD, DATA, PRIORITY):
- Step 1: Create new node with DATA and PRIORITY
- Step 2: Check if HEAD has lower priority. If true follow Steps 3-4 and end. Else goto Step 5.
- Step 3: NEW -> NEXT = HEAD
- Step 4: HEAD = NEW
- Step 5: Set TEMP to head of the list
- Step 6: While TEMP -> NEXT != NULL and TEMP -> NEXT -> PRIORITY > PRIORITY
- Step 7: TEMP = TEMP -> NEXT
[END OF LOOP]- Step 8: NEW -> NEXT = TEMP -> NEXT
- Step 9: TEMP -> NEXT = NEW
- Step 10: End
POP(HEAD):
- Step 1: Set the head of the list to the next node in the list. HEAD = HEAD -> NEXT.
- Step 2: Free the node at the head of the list
- Step 3: End
PEEK(HEAD):
- Step 1: Return HEAD -> DATA
- Step 2: End
Below is the implementation of the algorithm :
// C++ code to implement Priority Queue // using Linked List #include <bits/stdc++.h> using namespace std;
// Node typedef struct node
{ int data;
// Lower values indicate
// higher priority
int priority;
struct node* next;
} Node; // Function to create a new node Node* newNode( int d, int p)
{ Node* temp = (Node*) malloc ( sizeof (Node));
temp->data = d;
temp->priority = p;
temp->next = NULL;
return temp;
} // Return the value at head int peek(Node** head)
{ return (*head)->data;
} // Removes the element with the // highest priority from the list void pop(Node** head)
{ Node* temp = *head;
(*head) = (*head)->next;
free (temp);
} // Function to push according to priority void push(Node** head, int d, int p)
{ Node* start = (*head);
// Create new Node
Node* temp = newNode(d, p);
// Special Case: The head of list has
// lesser priority than new node. So
// insert newnode before head node
// and change head node.
if ((*head)->priority > p)
{
// Insert New Node before head
temp->next = *head;
(*head) = temp;
}
else
{
// Traverse the list and find a
// position to insert new node
while (start->next != NULL &&
start->next->priority < p)
{
start = start->next;
}
// Either at the ends of the list
// or at required position
temp->next = start->next;
start->next = temp;
}
} // Function to check is list is empty int isEmpty(Node** head)
{ return (*head) == NULL;
} // Driver code int main()
{ // Create a Priority Queue
// 7->4->5->6
Node* pq = newNode(4, 1);
push(&pq, 5, 2);
push(&pq, 6, 3);
push(&pq, 7, 0);
while (!isEmpty(&pq))
{
cout << " " << peek(&pq);
pop(&pq);
}
return 0;
} // This code is contributed by shivanisinghss2110 |
// C code to implement Priority Queue // using Linked List #include <stdio.h> #include <stdlib.h> // Node typedef struct node {
int data;
// Lower values indicate higher priority
int priority;
struct node* next;
} Node; // Function to Create A New Node Node* newNode( int d, int p)
{ Node* temp = (Node*) malloc ( sizeof (Node));
temp->data = d;
temp->priority = p;
temp->next = NULL;
return temp;
} // Return the value at head int peek(Node** head)
{ return (*head)->data;
} // Removes the element with the // highest priority from the list void pop(Node** head)
{ Node* temp = *head;
(*head) = (*head)->next;
free (temp);
} // Function to push according to priority void push(Node** head, int d, int p)
{ Node* start = (*head);
// Create new Node
Node* temp = newNode(d, p);
// Special Case: The head of list has lesser
// priority than new node. So insert new
// node before head node and change head node.
if ((*head)->priority > p) {
// Insert New Node before head
temp->next = *head;
(*head) = temp;
}
else {
// Traverse the list and find a
// position to insert new node
while (start->next != NULL &&
start->next->priority < p) {
start = start->next;
}
// Either at the ends of the list
// or at required position
temp->next = start->next;
start->next = temp;
}
} // Function to check is list is empty int isEmpty(Node** head)
{ return (*head) == NULL;
} // Driver code int main()
{ // Create a Priority Queue
// 7->4->5->6
Node* pq = newNode(4, 1);
push(&pq, 5, 2);
push(&pq, 6, 3);
push(&pq, 7, 0);
while (!isEmpty(&pq)) {
printf ( "%d " , peek(&pq));
pop(&pq);
}
return 0;
} |
// Java code to implement Priority Queue // using Linked List import java.util.* ;
class Solution
{ // Node static class Node {
int data;
// Lower values indicate higher priority
int priority;
Node next;
} static Node node = new Node();
// Function to Create A New Node static Node newNode( int d, int p)
{ Node temp = new Node();
temp.data = d;
temp.priority = p;
temp.next = null ;
return temp;
} // Return the value at head static int peek(Node head)
{ return (head).data;
} // Removes the element with the // highest priority from the list static Node pop(Node head)
{ Node temp = head;
(head) = (head).next;
return head;
} // Function to push according to priority static Node push(Node head, int d, int p)
{ Node start = (head);
// Create new Node
Node temp = newNode(d, p);
// Special Case: The head of list has lesser
// priority than new node. So insert new
// node before head node and change head node.
if ((head).priority > p) {
// Insert New Node before head
temp.next = head;
(head) = temp;
}
else {
// Traverse the list and find a
// position to insert new node
while (start.next != null &&
start.next.priority < p) {
start = start.next;
}
// Either at the ends of the list
// or at required position
temp.next = start.next;
start.next = temp;
}
return head;
} // Function to check is list is empty static int isEmpty(Node head)
{ return ((head) == null )? 1 : 0 ;
} // Driver code public static void main(String args[])
{ // Create a Priority Queue
// 7.4.5.6
Node pq = newNode( 4 , 1 );
pq =push(pq, 5 , 2 );
pq =push(pq, 6 , 3 );
pq =push(pq, 7 , 0 );
while (isEmpty(pq)== 0 ) {
System.out.printf( "%d " , peek(pq));
pq=pop(pq);
}
} } // This code is contributed // by Arnab Kundu |
# Python3 code to implement Priority Queue # using Singly Linked List # Class to create new node which includes # Node Data, and Node Priority class PriorityQueueNode:
def __init__( self , value, pr):
self .data = value
self .priority = pr
self . next = None
# Implementation of Priority Queue class PriorityQueue:
def __init__( self ):
self .front = None
# Method to check Priority Queue is Empty
# or not if Empty then it will return True
# Otherwise False
def isEmpty( self ):
return True if self .front = = None else False
# Method to add items in Priority Queue
# According to their priority value
def push( self , value, priority):
# Condition check for checking Priority
# Queue is empty or not
if self .isEmpty() = = True :
# Creating a new node and assigning
# it to class variable
self .front = PriorityQueueNode(value,
priority)
# Returning 1 for successful execution
return 1 else :
# Special condition check to see that
# first node priority value
if self .front.priority > priority:
# Creating a new node
newNode = PriorityQueueNode(value,
priority)
# Updating the new node next value
newNode. next = self .front
# Assigning it to self.front
self .front = newNode
# Returning 1 for successful execution
return 1
else :
# Traversing through Queue until it
# finds the next smaller priority node
temp = self .front
while temp. next :
# If same priority node found then current
# node will come after previous node
if priority < = temp. next .priority:
break
temp = temp. next
newNode = PriorityQueueNode(value,
priority)
newNode. next = temp. next
temp. next = newNode
# Returning 1 for successful execution
return 1 # Method to remove high priority item
# from the Priority Queue
def pop( self ):
# Condition check for checking
# Priority Queue is empty or not
if self .isEmpty() = = True :
return
else :
# Removing high priority node from
# Priority Queue, and updating front
# with next node
self .front = self .front. next
return 1
# Method to return high priority node
# value Not removing it
def peek( self ):
# Condition check for checking Priority
# Queue is empty or not
if self .isEmpty() = = True :
return
else :
return self .front.data
# Method to Traverse through Priority
# Queue
def traverse( self ):
# Condition check for checking Priority
# Queue is empty or not
if self .isEmpty() = = True :
return "Queue is Empty!"
else :
temp = self .front
while temp:
print (temp.data, end = " " )
temp = temp. next
# Driver code if __name__ = = "__main__" :
# Creating an instance of Priority
# Queue, and adding values
# 7 -> 4 -> 5 -> 6
pq = PriorityQueue()
pq.push( 4 , 1 )
pq.push( 5 , 2 )
pq.push( 6 , 3 )
pq.push( 7 , 0 )
# Traversing through Priority Queue
pq.traverse()
# Removing highest Priority item
# for priority queue
pq.pop()
# This code is contributed by himanshu kanojiya |
// C# code to implement Priority Queue // using Linked List using System;
class GFG
{ // Node public class Node
{ public int data;
// Lower values indicate
// higher priority
public int priority;
public Node next;
} public static Node node = new Node();
// Function to Create A New Node public static Node newNode( int d, int p)
{ Node temp = new Node();
temp.data = d;
temp.priority = p;
temp.next = null ;
return temp;
} // Return the value at head public static int peek(Node head)
{ return (head).data;
} // Removes the element with the // highest priority from the list public static Node pop(Node head)
{ Node temp = head;
(head) = (head).next;
return head;
} // Function to push according to priority public static Node push(Node head,
int d, int p)
{ Node start = (head);
// Create new Node
Node temp = newNode(d, p);
// Special Case: The head of list
// has lesser priority than new node.
// So insert new node before head node
// and change head node.
if ((head).priority > p)
{
// Insert New Node before head
temp.next = head;
(head) = temp;
}
else
{
// Traverse the list and find a
// position to insert new node
while (start.next != null &&
start.next.priority < p)
{
start = start.next;
}
// Either at the ends of the list
// or at required position
temp.next = start.next;
start.next = temp;
}
return head;
} // Function to check is list is empty public static int isEmpty(Node head)
{ return ((head) == null ) ? 1 : 0;
} // Driver code public static void Main( string [] args)
{ // Create a Priority Queue
// 7.4.5.6
Node pq = newNode(4, 1);
pq = push(pq, 5, 2);
pq = push(pq, 6, 3);
pq = push(pq, 7, 0);
while (isEmpty(pq) == 0)
{
Console.Write( "{0:D} " , peek(pq));
pq = pop(pq);
}
} } // This code is contributed by Shrikant13 |
<script> // JavaScript code to implement Priority Queue
// using Linked List
// Node
class Node
{
// Lower values indicate
// higher priority
constructor() {
this .data = 0;
this .priority = 0;
this .next = null ;
}
}
var node = new Node();
// Function to Create A New Node
function newNode(d, p) {
var temp = new Node();
temp.data = d;
temp.priority = p;
temp.next = null ;
return temp;
}
// Return the value at head
function peek(head) {
return head.data;
}
// Removes the element with the
// highest priority from the list
function pop(head) {
var temp = head;
head = head.next;
return head;
}
// Function to push according to priority
function push(head, d, p) {
var start = head;
// Create new Node
var temp = newNode(d, p);
// Special Case: The head of list
// has lesser priority than new node.
// So insert new node before head node
// and change head node.
if (head.priority > p)
{
// Insert New Node before head
temp.next = head;
head = temp;
}
else
{
// Traverse the list and find a
// position to insert new node
while (start.next != null && start.next.priority < p) {
start = start.next;
}
// Either at the ends of the list
// or at required position
temp.next = start.next;
start.next = temp;
}
return head;
}
// Function to check is list is empty
function isEmpty(head) {
return head == null ? 1 : 0;
}
// Driver code
// Create a Priority Queue
// 7.4.5.6
var pq = newNode(4, 1);
pq = push(pq, 5, 2);
pq = push(pq, 6, 3);
pq = push(pq, 7, 0);
while (isEmpty(pq) == 0) {
document.write(peek(pq) + " " );
pq = pop(pq);
}
// This code is contributed by rdtank.
</script>
|
7 4 5 6
Time Complexities and Comparison with Binary Heap:
peek() push() pop() ----------------------------------------- Linked List | O(1) O(n) O(1) | Binary Heap | O(1) O(Log n) O(Log n)