# Stacks and Queues

Stacks and Queues are linear data structures that differ primarily in how elements are added and removed. They are often implemented using Arrays or Linked Lists.

## 1. Stack (LIFO)

A **Stack** follows the **Last In, First Out** (LIFO) principle. The last element added is the first one to be removed.

*   **Analogy:** A stack of plates. You put a new plate on top, and you take the top plate off first. You can't take a plate from the middle or bottom easily.

### Operations
*   **Push:** Add an element to the top.
*   **Pop:** Remove the top element.
*   **Peek (Top):** View the top element without removing it.

### Use Cases
*   **Function Call Stack:** Tracking function calls and local variables (recursion).
*   **Undo/Redo:** Storing history of actions.
*   **Browser History:** Back button functionality.
*   **Syntax Parsing:** Checking for balanced parentheses `(( ))`.

## 2. Queue (FIFO)

A **Queue** follows the **First In, First Out** (FIFO) principle. The first element added is the first one to be removed.

*   **Analogy:** A line of people waiting for a bus. The person who arrived first gets on the bus first.

### Operations
*   **Enqueue:** Add an element to the back (rear).
*   **Dequeue:** Remove the element from the front.
*   **Peek (Front):** View the front element without removing it.

### Use Cases
*   **Task Scheduling:** Printer queues, CPU task scheduling.
*   **Breadth-First Search (BFS):** Exploring graph nodes layer by layer.
*   **Message Queues:** Handling asynchronous data between services (RabbitMQ).

## 3. Implementation (Python)

Python lists can be used as stacks. For queues, `collections.deque` is more efficient.

```python
# Stack Implementation
stack = []
stack.append(1) # Push
stack.append(2)
print(stack.pop()) # Pop -> 2

# Queue Implementation
from collections import deque
queue = deque()
queue.append(1) # Enqueue
queue.append(2)
print(queue.popleft()) # Dequeue -> 1
```

[[programming/arrays-and-lists]]
[[programming/arrays-vs-linked-lists]]
[[programming/algorithms]]