Queus.md

🔷 Understanding Queues

🔹 What is a Queue?

A Queue is a linear data structure that follows the First In, First Out (FIFO) principle.
The first element added is the first one removed.
Think of it like a line at a shop — the person who joins first is served first.

---

🔹 Real-World Analogy

• People waiting in a queue at a bank or supermarket
• Print jobs waiting in a printer queue
• Task scheduling in operating systems

---

🔹 Core Operations

Operation	Description	Example
enqueue(x)	Add element x to the end	queue.append(x)
dequeue()	Remove and return the front element	queue.pop(0)
peek() / front()	Get the front element without removing it	queue[0]
isEmpty()	Check if the queue is empty	len(queue) == 0

---

🔹 Code Examples

Python

# Queue implementation using list
queue = []
queue.append(1)   # enqueue
queue.append(2)
print(queue.pop(0))  # dequeue -> 1

---

JavaScript

// Queue implementation using array
let queue = [];
queue.push(1);   // enqueue
queue.push(2);
console.log(queue.shift()); // dequeue -> 1

---

🔹 Mini-Test

Question: What does FIFO mean? Answer: First In, First Out — the first element inserted is the first to be removed.

---

🔹 Practice Problem

Task: Implement a function to simulate a queue using two stacks.

Example

Input:

enqueue(1)
enqueue(2)
dequeue()
enqueue(3)
dequeue()

Output:

1
2

---

🔹 Python Implementation

class QueueUsingStacks:
    def __init__(self):
        self.stack1 = []
        self.stack2 = []

    def enqueue(self, x):
        self.stack1.append(x)

    def dequeue(self):
        if not self.stack2:
            while self.stack1:
                self.stack2.append(self.stack1.pop())
        if not self.stack2:
            raise IndexError("Queue is empty")
        return self.stack2.pop()

# Test
q = QueueUsingStacks()
q.enqueue(1)
q.enqueue(2)
print(q.dequeue())  # 1
q.enqueue(3)
print(q.dequeue())  # 2

---

🔹 JavaScript Implementation

class QueueUsingStacks {
  constructor() {
    this.stack1 = [];
    this.stack2 = [];
  }

  enqueue(x) {
    this.stack1.push(x);
  }

  dequeue() {
    if (this.stack2.length === 0) {
      while (this.stack1.length > 0) {
        this.stack2.push(this.stack1.pop());
      }
    }
    if (this.stack2.length === 0) throw new Error("Queue is empty");
    return this.stack2.pop();
  }
}

// Test
const q = new QueueUsingStacks();
q.enqueue(1);
q.enqueue(2);
console.log(q.dequeue()); // 1
q.enqueue(3);
console.log(q.dequeue()); // 2

---

🔹 Key Takeaways

• Queue = FIFO structure

• Enqueue → add to rear

• Dequeue → remove from front

• Used in:

  • Task scheduling
  • Breadth-first search (BFS)
  • Message queues
  • CPU job management

---