# 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

```// 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
```

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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