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‎01. Data Structures/03. Queue/Basic Operations/01. Queue using Array/Queue_Array.java

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import java.util.*;
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public class Queue_Array {
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public static void main(String[] args) {
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Scanner sc = new Scanner(System.in);
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System.out.println("Enter Size of Queue.");
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int size = sc.nextInt();
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Queue q = new Queue(size);
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boolean flag = true;
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int val = 0;
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while (flag) {
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System.out.println("1. Enqueue()");
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System.out.println("2. Dequeue()");
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System.out.println("3. Current Size of Queue");
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System.out.println("4. Peek()");
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System.out.println("5. View queue");
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System.out.println("6. Exit");
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System.out.println("Enter Choice");
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int choice = sc.nextInt();
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switch (choice) {
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case 1:
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System.out.println("Enter value");
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val = sc.nextInt();
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q.enqueue(val);
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break;
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case 2:
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System.out.println(q.dequeue());
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break;
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case 3:
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System.out.println(q.count());
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break;
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case 4:
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System.out.println(q.peek());
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break;
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case 5:
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q.viewQ();
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break;
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case 6:
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flag = false;
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break;
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default:
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System.out.println("Invalid Choice");
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}// switch
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System.out.println();
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} // while
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}
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}
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class Queue {
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private int front;
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private int rear;
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private int n;
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private int arr[];
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public Queue(int size) {
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this.n = size;
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arr = new int[n];
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front = -1;
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rear = -1;
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}
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public boolean isEmpty() {
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return (rear == -1 && front == -1);
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}
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public boolean isFull() {
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return (rear == n - 1);
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}
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public int count() {
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return rear - front + 1;
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}
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public void enqueue(int val) {
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if (isFull()) {
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System.out.println("Queue is Full");
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return;
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}
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else if (front == -1) {
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front = rear = 0;
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}
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else {
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rear++;
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}
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arr[rear] = val;
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}
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public int dequeue() {
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int x = 0;
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if (isEmpty()) {
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System.out.println("Queue is Empty");
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return -999;
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} else if (rear == front) {
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x = arr[front];
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front = rear = -1;
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return x;
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} else {
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x = arr[front];
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front++;
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return x;
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}
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}
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public void viewQ() {
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if (isEmpty()) {
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System.out.println("Queue is empty");
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return;
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}
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for (int i = front; i <= rear; i++) {
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System.out.print(arr[i] + " ");
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}
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}
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public int peek() {
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if (isEmpty()) {
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System.out.println("Queue is empty");
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return -999;
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}
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return arr[front];
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}
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}

‎01. Data Structures/03. Queue/Basic Operations/01. Queue using Array/README.md

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## **Definition:**
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Queue is a linear data structure which operates in a **First IN First OUT or Last IN Last OUT.** Queue is an abstract data type with a bounded (predefined) capacity. It is a simple data structure that allows adding and removing elements in a particular order.
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**The order is FIFO(First IN First OUT) or LILO(Last In Last Out).**
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**It is named queue as it behaves like a real-world queue for examples –**
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- queue(line) of cars in a single lane,
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- queue of people waiting at food counter etc.
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## **Standard Queue Operations:**
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- Enqueue() – Add item to the queue from the REAR.
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- Dequeue() – Remove item from the queue from the FRONT.
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- isFull() – Check if queue is full or not.
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- isEmpty() – Check if queue empty or not.
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- count() – Get number of items in the queue.
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- peek() – return front elemenet in the queue(line).
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<h1><img src="https://simplesnippets.tech/wp-content/uploads/2019/04/queue-data-structure-diagram.jpg" width=700/></h1>
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<!-- <br> -->
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<h3> <b>Step 1:</b> <a href="https://youtu.be/fbonDkYsKj0">Watch this introductory video on apna college</a>
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<h3><b>Step 2:</b> Visualize pseudocode</h3>
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<h3><img src="./pseudocode.png"/></h3>
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### **Step 3:** [See my code here in java](./Queue_Array.java)
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<h1 align="Center">Thank You</h1>

‎01. Data Structures/03. Queue/Basic Operations/01. Queue using Array/pseudocode.png

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‎01. Data Structures/03. Queue/Basic Operations/02. Queue using Linked List/Queue_LL.c

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#include <stdio.h>
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#include <malloc.h>
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struct Queue
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{
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int val;
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struct Queue *next;
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};
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typedef struct Queue N;
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N *front = NULL, *rear = NULL, *ptr;
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void Insert()
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{
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ptr = (N *)malloc(sizeof(N));
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printf("Enter value : ");
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scanf("%d",&ptr->val);
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ptr->next = NULL;
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if(front == NULL)
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{
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front = ptr;
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rear = ptr;
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front->next = NULL;
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rear->next = NULL;
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}
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else
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{
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rear->next = ptr;
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rear = ptr;
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rear->next = NULL;
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}
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}
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void Delete()
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{
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if(front == NULL)
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{
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printf("\nUnderflow\n");
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return;
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}
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ptr = front;
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front = front->next;
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free(ptr);
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}
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void Display()
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{
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ptr = front;
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while(ptr!=rear)
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{
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printf("%d ",ptr->val);
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ptr = ptr->next;
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}
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}
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int main()
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{
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int ch;
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do
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{
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printf("\n\tM.E.N.U.\n=======================\n");
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printf("\n1.....Insert.");
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printf("\n2.....Delete.");
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printf("\n3.....Display.");
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printf("\n4.....Exit.");
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printf("\nEnter your choice : ");
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scanf("%d", &ch);
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switch (ch)
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{
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case 1:
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Insert();
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break;
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case 2:
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Delete();
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break;
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case 3:
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Display();
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break;
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case 4:
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printf("\nEnd of Program\n");
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break;
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default:
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printf("\nInvalid Input!!\n");
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break;
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}
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} while (ch != 4);
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return 0;
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}

‎01. Data Structures/03. Queue/Basic Operations/02. Queue using Linked List/Queue_LinkedList.java

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import java.util.*;
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public class Queue_LinkedList {
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public static void main(String[] args) {
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Scanner sc = new Scanner(System.in);
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Queue_LL q = new Queue_LL();
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boolean flag = true;
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int val = 0;
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while (flag) {
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System.out.println("1. Enqueue()");
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System.out.println("2. Dequeue()");
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System.out.println("3. Current Size of Queue");
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System.out.println("4. Peek()");
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System.out.println("5. View Queue");
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System.out.println("6. Exit");
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System.out.println("Enter Choice");
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int choice = sc.nextInt();
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switch (choice) {
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case 1:
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System.out.println("Enter value");
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val = sc.nextInt();
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q.enqueue(val);
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break;
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case 2:
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System.out.println(q.dequeue());
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break;
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case 3:
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System.out.println(q.count());
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break;
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case 4:
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System.out.println(q.peek());
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break;
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case 5:
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q.viewQ();
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break;
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case 6:
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flag = false;
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break;
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default:
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System.out.println("invalid choice");
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}// switch
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System.out.println();
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} // while
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}
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}
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class Queue_LL {
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class Node {
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private int data;
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private Node next;
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public Node(int data) {
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this.data = data;
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next = null;
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}
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}
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private Node front;
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private Node rear;
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private int size;
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public Queue_LL() {
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front = null;
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rear = null;
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size = 0;
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}
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public boolean isEmpty() {
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return (front == null && rear == null);
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}
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public int count() {
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return size;
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}
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public void enqueue(int val) {
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Node n = new Node(val);
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if (isEmpty()) {
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front = rear = n;
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} else {
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rear.next = n;
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rear = n;
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}
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size++;
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}
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public int dequeue() {
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if (isEmpty()) {
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System.out.println("Queue is empty");
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return -999;
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} else if (front == rear) {
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Node delD = front;
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front = rear = null;
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size--;
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return delD.data;
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} else {
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Node delD = front;
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front = front.next;
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delD.next = null;
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size--;
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return delD.data;
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}
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}
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public int peek() {
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if (isEmpty()) {
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System.out.println("Queue is Empty");
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return -999;
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}
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return front.data;
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}
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public void viewQ() {
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if (isEmpty()) {
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System.out.println("Queue is Empty");
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return;
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}
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Node t = front;
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while (t != null) {
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System.out.print(t.data + " ");
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t = t.next;
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}
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}
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}

‎01. Data Structures/03. Queue/Basic Operations/02. Queue using Linked List/README.md

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### **Step 1:** [Visualize Queue using linklist here by Apna College in 6min only](https://youtu.be/Yi8Im4_eSQI)
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### **Step 2:** [Recall your Linklist knowledge and try to understand my code here.](./Queue_LinkedList.java)

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