Queue using linked list

 

What is a Queue?

  • A Queue is a linear data structure that follows FIFO (First In First Out) principle.

  • Insertion happens at the rear (end).

  • Deletion happens at the front (beginning).

Think of a queue like a line of people at a ticket counter:
First person enters first, leaves first.

How to Implement Queue using Linked List?

  • Use a linked list instead of an array.

  • Maintain two pointers:

    • front: Points to the first element.

    • rear: Points to the last element.

  • Each node contains:

    • Data

    • Pointer to next node

Algorithms for Queue using Linked List

Enqueue (Insert at the end)

1. Create a new node and assign value to its data field.

2. Set new node's next pointer to NULL.

3. If front == NULL (Queue is empty):

    a. Set front = new node

    b. Set rear = new node

4. Else:

    a. Set rear->next = new node

    b. Set rear = new node

5. End

Dequeue (Remove from Front)

Algorithm Dequeue():
1. If front == NULL:
    a. Queue is empty. (Underflow condition)
    b. Return error or -1
2. Else:
    a. Save front node temporarily (temp = front)
    b. Store temp->data to return later
    c. Move front to front->next
    d. If front == NULL after moving:
         i. Set rear = NULL (Queue became empty)
    e. Free the temporary node (temp)
    f. Return the stored data
3. End

3. Display Queue

1. If front == NULL:
    a. Print "Queue is empty"
2. Else:
    a. Start from temp = front
    b. While temp is not NULL:
         i. Print temp->data
         ii. Move temp to temp->next
3. End



C program - Queue using Linked List

#include<stdio.h>
#include<conio.h>
#include <stdlib.h>
struct Node
{
    int data;
    struct Node *next;
}*front=NULL,*rear=NULL;
void enqueue()
{
    struct Node *newnode=(struct Node*)malloc(sizeof(struct Node));
    printf("Enter data:");
    scanf("%d",&newnode->data);
    newnode->next=NULL;
    if(front==NULL)
    {
    front=newnode;
    rear=newnode;
    }
    else
    {
    rear->next=newnode;
    rear=newnode;
    }
    }
int dequeue()
{
    if (front==NULL)
    {printf("Queue is empty...underflow");return -1;}
    else
    {
    struct Node *temp;
    temp=front;
    int rv=front->data;
    front=front->next;
    if(front==NULL) rear=NULL;
    free(temp);
    return(rv);
    }
}
void displayQ()
{
    struct Node *temp=front;
    while(temp!=NULL)
    {
    printf(" %d---->",temp->data);
    temp=temp->next;
    }
    printf(" NULL");
}
void main()
{
    int ch;

do
{
printf("\nMenu\n----\n1.Enqueue\n2.Dequeue\n3.Display\n4.Exit");
printf("\nEnter the choice:");
scanf("%d",&ch);
switch(ch)
{
case 1:
    enqueue();
    displayQ();
    break;
case 2:
    int data;
    data=dequeue();
    if(data!=-1)
        printf("The dequed data is:%d\n",data);
    displayQ();
    break;
case 3:
    displayQ();
    break;
case 4:
    exit(0);
default:
    printf("There is no such operation:");
    }
  }
while(1);
}


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