Name some Queue implementations and compare them by efficiency of operations

A queue is a linear data structure in which insertion is done from one end called the rear end and deletion is done from another end called the front end. It follows FIFO (First In First Out) approach. The insertion in the queue is called enqueue operation and deletion in the queue is called dequeue operation. 

A queue can be implemented in two ways:

• Array implementation of queue
• Linked List implementation of queue

Array implementation of the queue:

For the array implementation of the queue, we have to take an array of size n. We also use two pointers front and rear to keep track of the front element and the last position where a new element can be inserted respectively. All the functionalities are satisfied by using these two pointers. For more details about array implementation of a queue refer to this link.

Below is the code for array implementation of a queue.

C++

// C++ program to implement queue using array

#include <bits/stdc++.h>
using namespace std;

// Structure of a queue
struct Queue {
    int rear, front, s;
    int* q;


    Queue(int c)
    {
        front = rear = 0;
        s = c;
        q = new int;
    }
    ~Queue() { delete[] q; }

    // Function to insert element at
    // the rear end of the queue
    void enqueue(int data)
    {
        if (rear == s)
            cout << "Queue is full\n";
        else {
            q[rear] = data;
            rear++;
        }
        return;
    }

    // Function to delete element at
    // the front end of the queue
    void dequeue()
    {
        if (rear == front)
            cout << "Queue is empty\n";
        else
            front++;
    }

    // Function to display the queue
    void display()
    {
        if (rear == front)
            cout << "Queue is empty\n";
        else
            for (int i = front; i < rear; i++) {
                cout << q[i] << " ";
            }
    }
};

// Driver code
int main()
{
    Queue p(3);
    p.enqueue(10);
    p.enqueue(20);
    p.enqueue(30);
    p.display();
    cout << "\nAfter two deletion\n";
    p.dequeue();
    p.dequeue();
    p.display();
    return 0;
}

Java

/*package whatever //do not write package name here */
import java.io.*;

class GFG {

  // Class of a queue
  static class Queue {
    int rear, front, s;
    int q[];

    // Constructor of a queue
    Queue(int c)
    {
      front = 0;
      rear = 0;
      s = c;
      q = new int[s];
    }

    // Function to insert element at
    // the rear end of the queue
    void enqueue(int data)
    {
      if (rear == s)
        System.out.println("Queue is full");
      else {
        q[rear] = data;
        rear++;
      }
      return;
    }

    // Function to delete element at
    // the front end of the queue
    void dequeue()
    {
      if (rear == front)
        System.out.println("Queue is empty");
      else
        front++;
    }

    // Function to display the queue
    void display()
    {
      if (rear == front)
        System.out.println("Queue is empty");
      else
        for (int i = front; i < rear; i++) {
          System.out.print(q[i]+" ");
        }
    }

  }

  public static void main (String[] args) {
    Queue p = new Queue(3);
    p.enqueue(10);
    p.enqueue(20);
    p.enqueue(30);
    p.display();
    System.out.println("\nAfter two deletion");
    p.dequeue();
    p.dequeue();
    p.display();
  }
}

// This code is contributed by aadityaburujwale.

Python3

# Structure of a queue
class Queue:

      # Constructor of a queue
    def __init__(self, c):
        self.rear = 0
        self.front = 0
        self.s = c
        self.q = [0] * c

    # Function to insert element at
    # the rear end of the queue
    def enqueue(self, data):
        if self.rear == self.s:
            print("Queue is full")
        else:
            self.q[self.rear] = data
            self.rear += 1

    # Function to delete element at
    # the front end of the queue
    def dequeue(self):
        if self.rear == self.front:
            print("Queue is empty")
        else:
            self.front += 1

    # Function to display the queue
    def display(self):
        if self.rear == self.front:
            print("Queue is empty")
        else:
            for i in range(self.front, self.rear):
                print(self.q[i], end=" ")
            print()

# Driver code
if __name__ == "__main__":
    p = Queue(3)
    p.enqueue(10)
    p.enqueue(20)
    p.enqueue(30)
    p.display()
    print("After two deletion")
    p.dequeue()
    p.dequeue()
    p.display()

# This code is contributed by akashish__

C#

// C# program to implement queue using array

using System;

public class GFG {

  // Class of a queue
  class Queue {
    public int rear, front, s;
    public int[] q;

    // Constructor of a queue
    public Queue(int c)
    {
      front = 0;
      rear = 0;
      s = c;
      q = new int[s];
    }

    // Function to insert element at
    // the rear end of the queue
    public void enqueue(int data)
    {
      if (rear == s)
        Console.WriteLine("Queue is full");
      else {
        q[rear] = data;
        rear++;
      }
      return;
    }

    // Function to delete element at
    // the front end of the queue
    public void dequeue()
    {
      if (rear == front)
        Console.WriteLine("Queue is empty");
      else
        front++;
    }

    // Function to display the queue
    public void display()
    {
      if (rear == front)
        Console.WriteLine("Queue is empty");
      else
        for (int i = front; i < rear; i++) {
          Console.Write(q[i] + " ");
        }
    }
  }

  static public void Main()
  {

    Queue p = new Queue(3);
    p.enqueue(10);
    p.enqueue(20);
    p.enqueue(30);
    p.display();
    Console.WriteLine("\nAfter two deletion");
    p.dequeue();
    p.dequeue();
    p.display();
  }
}

// This code is contributed by lokesh.

Javascript

// JS program to implement queue using array

// Structure of a queue
class Queue {
    constructor() {
        this.rear = 0;
        this.front = 0;
        this.q = new Array;
    }

    // Function to insert element at
    // the rear end of the queue
    enqueue(data) {
        if (this.rear == 3)
            console.log("Queue is full");
        else {
            this.q[this.rear] = data;
            this.rear++;
        }
        return;
    }

    // Function to delete element at
    // the front end of the queue
    dequeue() {
        if (this.rear == this.front)
            console.log("Queue is empty");
        else
            this.front++;
    }

    // Function to display the queue
    display() {
        if (this.rear == this.front)
            console.log("Queue is empty");
        else
            for (let i = this.front; i < this.rear; i++) {
                console.log(this.q[i], " ");
            }
    }
};


// Driver code
let p = new Queue;
p.enqueue(10);
p.enqueue(20);
p.enqueue(30);
p.display();
console.log("After two deletion");
p.dequeue();
p.dequeue();
p.display();

// This code is contributed by adityamaharshi21

Output

10 20 30 
After two deletion
30 

Time Complexity:

Insertion: O(1)
Deletion: O(1)
Searching: O(n)

Space Complexity: O(n)

Linked List Implementation of the queue:

For implementing a queue using linked list we don’t need to know the size beforehand like array. The dynamic property of linked list allows queue to grow to any size. In case of a linked list also we use two pointers front and rear that perform the same task as in array. For more details about linked list implementation refer to this link.

Below is the code for linked list implementation of queue.

C++

// Program to implement queue using linked list

#include <bits/stdc++.h>
using namespace std;

// Structure of a queue node
struct Qnode {
    int d;
    struct Qnode* next;
};

// Structure of a queue
struct Q {
    struct Qnode *front, *rear;
};

// Function to create a new node
struct Qnode* newNode(int k)
{
    struct Qnode* t
        = (struct Qnode*)malloc(sizeof(struct Qnode));
    t->d = k;
    t->next = NULL;
    return t;
}

// Function to create a queue
struct Q* createQ()
{
    struct Q* q = (struct Q*)malloc(sizeof(struct Q));
    q->front = q->rear = NULL;
    return q;
}

// Function to enqueue a new value
void enqueue(struct Q* q, int data)
{
    struct Qnode* t = newNode(data);
    if (q->rear == NULL) {
        q->front = q->rear = t;
        return;
    }
    q->rear->next = t;
    q->rear = t;
}

// Function for mimplementing deque
void dequeue(struct Q* q)
{
    if (q->front == NULL)
        return;
    struct Qnode* t = q->front;
    q->front = q->front->next;
    if (q->front == NULL)
        q->rear = NULL;
    free(t);
}

// Driver code
int main()
{
    struct Q* q = createQ();
    enqueue(q, 10);
    enqueue(q, 20);
    enqueue(q, 30);
    dequeue(q);

    cout << "Queue front  " << q->front->d;
    cout << "\nQueue rear  " << q->rear->d;
    return 0;
}

Java

class Qnode {
    int d;
    Qnode next;
}

class Q {
    Qnode front, rear;
    Q() {
        front = rear = null;
    }
}

class Main {
    static Qnode newNode(int k) {
        Qnode t = new Qnode();
        t.d = k;
        t.next = null;
        return t;
    }

    static Q createQ() {
        Q q = new Q();
        return q;
    }

    static void enqueue(Q q, int data) {
        Qnode t = newNode(data);
        if (q.rear == null) {
            q.front = q.rear = t;
            return;
        }
        q.rear.next = t;
        q.rear = t;
    }

    static void dequeue(Q q) {
        if (q.front == null) {
            return;
        }
        Qnode t = q.front;
        q.front = q.front.next;
        if (q.front == null) {
            q.rear = null;
        }
        t = null;
    }

    public static void main(String[] args) {
        Q q = createQ();
        enqueue(q, 10);
        enqueue(q, 20);
        enqueue(q, 30);
        dequeue(q);

        System.out.println("Queue front: " + q.front.d);
        System.out.println("Queue rear: " + q.rear.d);
    }
}

Python

class Qnode:
    def __init__(self, d):
        self.d = d
        self.next = None

class Q:
    def __init__(self):
        self.front = None
        self.rear = None

def newNode(k):
    t = Qnode(k)
    return t

def createQ():
    q = Q()
    return q

def enqueue(q, data):
    t = newNode(data)
    if q.rear is None:
        q.front = q.rear = t
        return
    q.rear.next = t
    q.rear = t

def dequeue(q):
    if q.front is None:
        return
    t = q.front
    q.front = q.front.next
    if q.front is None:
        q.rear = None
    t = None

q = createQ()
enqueue(q, 10)
enqueue(q, 20)
enqueue(q, 30)
dequeue(q)

print("Queue front: ", q.front.d)
print("Queue rear: ", q.rear.d)

C#

public class Qnode
{
    public int d;
    public Qnode next;

    public Qnode(int val)
    {
        d = val;
        next = null;
    }
}

public class Q
{
    public Qnode front, rear;

    public Q()
    {
        front = rear = null;
    }
}

public class MainClass
{
    public static Qnode newNode(int k)
    {
        Qnode t = new Qnode(k);
        return t;
    }

    public static Q createQ()
    {
        Q q = new Q();
        return q;
    }

    public static void enqueue(Q q, int data)
    {
        Qnode t = newNode(data);
        if (q.rear == null)
        {
            q.front = q.rear = t;
            return;
        }
        q.rear.next = t;
        q.rear = t;
    }

    public static void dequeue(Q q)
    {
        if (q.front == null)
        {
            return;
        }
        Qnode t = q.front;
        q.front = q.front.next;
        if (q.front == null)
        {
            q.rear = null;
        }
        t = null;
    }

    public static void Main(string[] args)
    {
        Q q = createQ();
        enqueue(q, 10);
        enqueue(q, 20);
        enqueue(q, 30);
        dequeue(q);

        System.Console.WriteLine("Queue front: " + q.front.d);
        System.Console.WriteLine("Queue rear: " + q.rear.d);
    }
}

// This code is contributed by factworx412.

Javascript

// Program to implement queue using linked list

class QNode {
    constructor(data) {
    this.d = data;
    this.next = null;
    }
    }

    class Q {
    constructor() {
    this.front = this.rear = null;
    }
    }

    const newNode = data => new QNode(data);

    const createQ = () => new Q();

    const enqueue = (q, data) => {
    const t = newNode(data);
    if (q.rear === null) {
    q.front = q.rear = t;
    return;
    }
    q.rear.next = t;
    q.rear = t;
    };

    const dequeue = q => {
    if (q.front === null) return;
    const t = q.front;
    q.front = q.front.next;
    if (q.front === null) q.rear = null;
    };

    // Driver code
    const q = createQ();
    enqueue(q, 10);
    enqueue(q, 20);
    enqueue(q, 30);
    dequeue(q);

    console.log("Queue front: ", q.front.d);
    console.log("Queue rear: ", q.rear.d);

    // This code is contributed by aadityamaharshi21.

Output

Queue front  20
Queue rear  30

Time complexity: The time complexity of enqueue and dequeue operations in a queue implemented using linked list is O(1). This is because, in a linked list, insertion and deletion operations are performed in constant time.

Space complexity: The space complexity of a queue implemented using linked list is O(n), where n is the number of elements in the queue. This is because we need to allocate memory for each element in the queue and the size of the queue increases or decreases as elements are added or removed.

Note: For easy understanding only the enqueue() and deque() functionalities are shown here. For detailed implementation you can check the links provided for implementation.

Comparison:

Queue Operations	Array Implementation		Linked-List Implementation
	Time Complexity	Space Complexity	Time Complexity	Space Complexity
Enqueue	O (1)	O (1)	O (1)	O (1)
Dequeue	O (1)	O (1)	O (1)	O (1)
IsFull	O (1)	O (1)	O (N)	O (1)
IsEmpty	O (1)	O (1)	O (1)	O (1)
Peek	O (1)	O (1)	O (1)	O (1)