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.

Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

C

// A C program to demonstrate linked list based implementation of queue 
#include <stdlib.h> 
#include <stdio.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 ths 
// 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

// 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 ths 
//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 

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 ths 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()) 
     

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.