Queue – Linked List Implementation

Last Updated : 12 Jan, 2023

In this article, the Linked List implementation of the queue data structure is discussed and implemented. Print ‘-1’ if the queue is empty.

Approach: To solve the problem follow the below idea:

we maintain two pointers, front, and rear. The front points to the first item of the queue and rear points to the last item.

enQueue(): This operation adds a new node after the rear and moves the rear to the next node.

deQueue(): This operation removes the front node and moves the front to the next node.

Follow the below steps to solve the problem:

Create a class QNode with data members integer data and QNode* next
- A parameterized constructor that takes an integer x value as a parameter and sets data equal to x and next as NULL
Create a class Queue with data members QNode front and rear
Enqueue Operation with parameter x:
- Initialize QNode* temp with data = x
- If the rear is set to NULL then set the front and rear to temp and return(Base Case)
- Else set rear next to temp and then move rear to temp
Dequeue Operation:
- If the front is set to NULL return(Base Case)
- Initialize QNode temp with front and set front to its next
- If the front is equal to NULL then set the rear to NULL
- Delete temp from the memory

Below is the Implementation of the above approach:

C++

// C++ program for the above approach
 
#include <bits/stdc++.h>
using namespace std;
 
struct QNode {
    int data;
    QNode* next;
    QNode(int d)
    {
        data = d;
        next = NULL;
    }
};
 
struct Queue {
    QNode *front, *rear;
    Queue() { front = rear = NULL; }
 
    void enQueue(int x)
    {
 
        // Create a new LL node
        QNode* temp = new QNode(x);
 
        // If queue is empty, then
        // new node is front and rear both
        if (rear == NULL) {
            front = rear = temp;
            return;
        }
 
        // Add the new node at
        // the end of queue and change rear
        rear->next = temp;
        rear = temp;
    }
 
    // Function to remove
    // a key from given queue q
    void deQueue()
    {
        // If queue is empty, return NULL.
        if (front == NULL)
            return;
 
        // Store previous front and
        // move front one node ahead
        QNode* temp = front;
        front = front->next;
 
        // If front becomes NULL, then
        // change rear also as NULL
        if (front == NULL)
            rear = NULL;
 
        delete (temp);
    }
};
 
// Driver code
int main()
{
 
    Queue q;
    q.enQueue(10);
    q.enQueue(20);
    q.deQueue();
    q.deQueue();
    q.enQueue(30);
    q.enQueue(40);
    q.enQueue(50);
    q.deQueue();
    cout << "Queue Front : " << ((q.front != NULL) ? (q.front)->data : -1)<< endl;
    cout << "Queue Rear : " << ((q.rear != NULL) ? (q.rear)->data : -1);
}
// This code is contributed by rathbhupendra

C

// A C program to demonstrate linked list based
// implementation of queue
#include <stdio.h>
#include <stdlib.h>
 
// A linked list (LL) node to store a queue entry
struct QNode {
    int key;
    struct QNode* next;
};
 
// The queue, front stores the front node of LL and rear
// stores the last node of LL
struct Queue {
    struct QNode *front, *rear;
};
 
// A utility function to create a new linked list node.
struct QNode* newNode(int k)
{
    struct QNode* temp
        = (struct QNode*)malloc(sizeof(struct QNode));
    temp->key = k;
    temp->next = NULL;
    return temp;
}
 
// A utility function to create an empty queue
struct Queue* createQueue()
{
    struct Queue* q
        = (struct Queue*)malloc(sizeof(struct Queue));
    q->front = q->rear = NULL;
    return q;
}
 
// The function to add a key k to q
void enQueue(struct Queue* q, int k)
{
    // Create a new LL node
    struct QNode* temp = newNode(k);
 
    // If queue is empty, then new node is front and rear
    // both
    if (q->rear == NULL) {
        q->front = q->rear = temp;
        return;
    }
 
    // Add the new node at the end of queue and change rear
    q->rear->next = temp;
    q->rear = temp;
}
 
// Function to remove a key from given queue q
void deQueue(struct Queue* q)
{
    // If queue is empty, return NULL.
    if (q->front == NULL)
        return;
 
    // Store previous front and move front one node ahead
    struct QNode* temp = q->front;
 
    q->front = q->front->next;
 
    // If front becomes NULL, then change rear also as NULL
    if (q->front == NULL)
        q->rear = NULL;
 
    free(temp);
}
 
// Driver code
int main()
{
    struct Queue* q = createQueue();
    enQueue(q, 10);
    enQueue(q, 20);
    deQueue(q);
    deQueue(q);
    enQueue(q, 30);
    enQueue(q, 40);
    enQueue(q, 50);
    deQueue(q);
    printf("Queue Front : %d \n", ((q->front != NULL) ? (q->front)->key : -1));
    printf("Queue Rear : %d", ((q->rear != NULL) ? (q->rear)->key : -1));
    return 0;
}

Java

// Java program for linked-list implementation of queue
 
// A linked list (LL) node to store a queue entry
class QNode {
    int key;
    QNode next;
 
    // constructor to create a new linked list node
    public QNode(int key)
    {
        this.key = key;
        this.next = null;
    }
}
 
// A class to represent a queue
// The queue, front stores the front node of LL and rear
// stores the last node of LL
class Queue {
    QNode front, rear;
 
    public Queue() { this.front = this.rear = null; }
 
    // Method to add an key to the queue.
    void enqueue(int key)
    {
 
        // Create a new LL node
        QNode temp = new QNode(key);
 
        // If queue is empty, then new node is front and
        // rear both
        if (this.rear == null) {
            this.front = this.rear = temp;
            return;
        }
 
        // Add the new node at the end of queue and change
        // rear
        this.rear.next = temp;
        this.rear = temp;
    }
 
    // Method to remove an key from queue.
    void dequeue()
    {
        // If queue is empty, return NULL.
        if (this.front == null)
            return;
 
        // Store previous front and move front one node
        // ahead
        QNode temp = this.front;
        this.front = this.front.next;
 
        // If front becomes NULL, then change rear also as
        // NULL
        if (this.front == null)
            this.rear = null;
    }
}
 
// Driver code
public class Test {
    public static void main(String[] args)
    {
        Queue q = new Queue();
        q.enqueue(10);
        q.enqueue(20);
        q.dequeue();
        q.dequeue();
        q.enqueue(30);
        q.enqueue(40);
        q.enqueue(50);
        q.dequeue();
        System.out.println("Queue Front : " + ((q.front != null) ? (q.front).key : -1));
        System.out.println("Queue Rear : " + ((q.rear != null) ? (q.rear).key : -1));
    }
}
// This code is contributed by Gaurav Miglani

Python3

# Python3 program to demonstrate linked list
# based implementation of queue
 
# A linked list (LL) node
# to store a queue entry
 
 
class Node:
 
    def __init__(self, data):
        self.data = data
        self.next = None
 
# A class to represent a queue
 
# The queue, front stores the front node
# of LL and rear stores the last node of LL
 
 
class Queue:
 
    def __init__(self):
        self.front = self.rear = None
 
    def isEmpty(self):
        return self.front == None
 
    # Method to add an item to the queue
    def EnQueue(self, item):
        temp = Node(item)
 
        if self.rear == None:
            self.front = self.rear = temp
            return
        self.rear.next = temp
        self.rear = temp
 
    # Method to remove an item from queue
    def DeQueue(self):
 
        if self.isEmpty():
            return
        temp = self.front
        self.front = temp.next
 
        if(self.front == None):
            self.rear = None
 
 
# Driver Code
if __name__ == '__main__':
    q = Queue()
    q.EnQueue(10)
    q.EnQueue(20)
    q.DeQueue()
    q.DeQueue()
    q.EnQueue(30)
    q.EnQueue(40)
    q.EnQueue(50)
    q.DeQueue()
    print("Queue Front : " + str(q.front.data if q.front != None else -1))
    print("Queue Rear : " + str(q.rear.data if q.rear != None else -1))

C#

// C# program for linked-list
// implementation of queue
using System;
 
// A linked list (LL) node to
// store a queue entry
class QNode {
    public int key;
    public QNode next;
 
    // constructor to create
    // a new linked list node
    public QNode(int key)
    {
        this.key = key;
        this.next = null;
    }
}
 
// A class to represent a queue The queue,
// front stores the front node of LL and
// rear stores the last node of LL
class Queue {
    public QNode front, rear;
 
    public Queue() { this.front = this.rear = null; }
 
    // Method to add an key to the queue.
    public void enqueue(int key)
    {
 
        // Create a new LL node
        QNode temp = new QNode(key);
 
        // If queue is empty, then new
        // node is front and rear both
        if (this.rear == null) {
            this.front = this.rear = temp;
            return;
        }
 
        // Add the new node at the
        // end of queue and change rear
        this.rear.next = temp;
        this.rear = temp;
    }
 
    // Method to remove an key from queue.
    public void dequeue()
    {
        // If queue is empty, return NULL.
        if (this.front == null)
            return;
 
        // Store previous front and
        // move front one node ahead
        this.front = this.front.next;
 
        // If front becomes NULL,
        // then change rear also as NULL
        if (this.front == null)
            this.rear = null;
    }
}
 
// Driver code
public class Test {
    public static void Main(String[] args)
    {
        Queue q = new Queue();
        q.enqueue(10);
        q.enqueue(20);
        q.dequeue();
        q.dequeue();
        q.enqueue(30);
        q.enqueue(40);
        q.enqueue(50);
        q.dequeue();
        Console.WriteLine("Queue Front : " + ((q.front != null) ? (q.front).key : -1));
        Console.WriteLine("Queue Rear : " + ((q.rear != null) ? (q.rear).key : -1));
    }
}
 
// This code has been contributed by Rajput-Ji

Javascript

<script>
// JavaScript program for linked-list implementation of queue
class QNode
{
    constructor(key)
    {
        this.key = key;
        this.next = null;
    }
}
 
let front = null, rear = null;
 
function enqueue(key)
{
    // Create a new LL node
        let temp = new QNode(key);
   
        // If queue is empty, then new node is front and rear both
        if (rear == null) {
            front = rear = temp;
            return;
        }
   
        // Add the new node at the end of queue and change rear
        rear.next = temp;
        rear = temp;
}
 
 
function dequeue()
{
    // If queue is empty, return NULL.
        if (front == null)
            return;
   
        // Store previous front and move front one node ahead
        let temp = front;
        front = front.next;
   
        // If front becomes NULL, then change rear also as NULL
        if (front == null)
            rear = null;
}
 
 
enqueue(10);
enqueue(20);
dequeue();
dequeue();
enqueue(30);
enqueue(40);
enqueue(50);
dequeue();
document.write("Queue Front : " +  ((front != null) ? (front).key : -1) +"<br>");
document.write("Queue Rear : " +  ((rear != null) ? (rear).key : -1) +"<br>");
 
 
// This code is contributed by avanitrachhadiya2155
</script>

Output

Queue Front : 40
Queue Rear : 50

Time Complexity: O(1), The time complexity of both operations enqueue() and dequeue() is O(1) as it only changes a few pointers in both operations
Auxiliary Space: O(1), The auxiliary Space of both operations enqueue() and dequeue() is O(1) as constant extra space is required

Suggest improvement

Introduction and Array Implementation of Queue

Applications, Advantages and Disadvantages of Queue

Share your thoughts in the comments

Queue implementation in different languages

Some question related to Queue implementation

Easy problems on Queue

Intermediate problems on Queue

Hard problems on Queue