Queue | Set 2 (Linked List Implementation)

In the previous post, we introduced Queue and discussed array implementation. In this post, linked list implementation is discussed. The following two main operations must be implemented efficiently.

In a Queue data structure, we maintain two pointers, front and rear. The front points the first item of queue and rear points to last item.

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

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

C++

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// A CPP program to demonstrate linked list
// based implementation of queue
#include <bits/stdc++.h>
using namespace std;
  
// A linked list (LL) node to store a queue entry
class QNode {
public:
    int key;
    QNode* next;
};
  
// The queue, front stores the front node
// of LL and rear stores the last node of LL
class Queue {
public:
    QNode *front, *rear;
};
  
// A utility function to create
// a new linked list node.
QNode* newNode(int k)
{
    QNode* temp = new QNode();
    temp->key = k;
    temp->next = NULL;
    return temp;
}
  
// A utility function to create an empty queue
Queue* createQueue()
{
    Queue* q = new Queue();
    q->front = q->rear = NULL;
    return q;
}
  
// The function to add a key k to q
void enQueue(Queue* q, int k)
{
    // Create a new LL node
    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
QNode* deQueue(Queue* q)
{
    // If queue is empty, return NULL.
    if (q->front == NULL)
        return NULL;
  
    // Store previous front and
    // move front one node ahead
    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;
    return temp;
}
  
// Driver code
int main()
{
    Queue* q = createQueue();
    enQueue(q, 10);
    enQueue(q, 20);
    deQueue(q);
    deQueue(q);
    enQueue(q, 30);
    enQueue(q, 40);
    enQueue(q, 50);
    QNode* n = deQueue(q);
    if (n != NULL)
        cout << "Dequeued item is " << n->key;
    return 0;
}
  
// This code is contributed by rathbhupendra

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C





// A C program to demonstrate linked list based implementation of queue

#include 

#include 


// 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

struct QNode* deQueue(struct Queue* q)

{

    // If queue is empty, return NULL.

    if (q->front == NULL)

        return NULL;


    // 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;

    return temp;

}


// Driver Program to test anove functions

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);

    struct QNode* n = deQueue(q);

    if (n != NULL)

        printf(“Dequeued item is %d”, n->key);

    return 0;

}



Java














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// 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. 


    QNode dequeue() 


    { 


        // If queue is empty, return NULL. 


        if (this.front == null) 


            return null; 


  


        // 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; 


        return temp; 


    } 


} 


  


// Driver class 


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); 


  


        System.out.println("Dequeued item is " + q.dequeue().key); 


    } 


} 


// This code is contributed by Gaurav Miglani 



















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Python3














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# 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


        return str(temp.data) 


  


# 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) 


      


    print("Dequeued item is " + q.DeQueue()) 


     



















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C#














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// 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 { 


    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 QNode dequeue() 


    { 


        // If queue is empty, return NULL. 


        if (this.front == null) 


            return null; 


  


        // 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; 


        return temp; 


    } 


} 


  


// 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); 


  


        Console.WriteLine("Dequeued item is " + q.dequeue().key); 


    } 


} 


  


// This code has been contributed by Rajput-Ji 



















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Output: 


Dequeued item is 30


Time Complexity: Time complexity of both operations enqueue() and dequeue() is O(1) as we only change few pointers in both operations. There is no loop in any of the operations.


 



 


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