Stacks.md

📘 Understanding Stacks in Detail

A Stack is a linear data structure that follows the LIFO (Last In, First Out) principle — the last element added is the first one to be removed. Think of a stack as a pile of plates: you can only remove the top plate first.

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🧠 Core Concept

• LIFO Principle → Last element pushed is the first to pop out.
• Operations occur only at one end (the top).
• Used in function calls, undo/redo, expression evaluation, backtracking, and browser history.

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🧩 Basic Stack Operations

Operation	Description	Time Complexity
push(x)	Add an element x on top of the stack	O(1)
pop()	Remove the top element	O(1)
peek() or top()	View the top element without removing it	O(1)
isEmpty()	Check if the stack is empty	O(1)
size()	Return the number of elements	O(1)

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🏗️ Stack Implementation Examples

1. Using Array (Static Stack)

class Stack {
  constructor() {
    this.items = [];
  }

  push(element) {
    this.items.push(element);
  }

  pop() {
    if (this.isEmpty()) return "Underflow";
    return this.items.pop();
  }

  peek() {
    return this.items[this.items.length - 1];
  }

  isEmpty() {
    return this.items.length === 0;
  }

  size() {
    return this.items.length;
  }
}