Introduction to Stacks

Stacks are everywhere in programming: function calls, undo/redo, expression evaluation. But when should you reach for a stack in your own code? After solving many problems, I've learned to recognize when stacks are the perfect tool.

What Is a Stack?

A stack is a Last In, First Out (LIFO) data structure. Think of it like a stack of plates. The last plate you put on top is the first one you take off. Simple, right? But this simple property makes stacks incredibly useful.

My First Stack Implementation

When I first implemented a stack, I kept it simple:

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

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

  pop() {
    if (this.isEmpty()) {
      return "Stack is empty";
    }
    return this.items.pop();
  }

  peek() {
    if (this.isEmpty()) {
      return "Stack is empty";
    }
    return this.items[this.items.length - 1];
  }

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

When I Use Stacks

I've found stacks are perfect for:

Matching Problems: When I need to match opening and closing brackets, parentheses, or tags:

function isValidParentheses(s) {
  const stack = [];
  const pairs = { "(": ")", "[": "]", "{": "}" };

  for (let char of s) {
    if (pairs[char]) {
      stack.push(char); // Opening bracket
    } else {
      if (stack.length === 0) return false;
      const last = stack.pop();
      if (pairs[last] !== char) return false;
    }
  }

  return stack.length === 0;
}

Reversing Order: When I need to process things in reverse:

function reverseString(str) {
  const stack = [];
  for (let char of str) {
    stack.push(char);
  }

  let reversed = "";
  while (!stack.isEmpty()) {
    reversed += stack.pop();
  }
  return reversed;
}

Tracking State: When I need to remember previous states (like undo/redo):

class Editor {
  constructor() {
    this.content = "";
    this.undoStack = [];
  }

  type(text) {
    this.undoStack.push(this.content); // Save state
    this.content += text;
  }

  undo() {
    if (this.undoStack.length > 0) {
      this.content = this.undoStack.pop();
    }
  }
}

The "Stack Overflow" Joke

Fun fact: when I first learned about stack overflow errors, I thought it was related to the website! Turns out, stack overflow happens when you try to push to a full stack (or when recursion goes too deep). The website name is actually a programming joke. 😄

What I Learned

Stacks have become one of my favorite data structures because:

• They're simple but powerful
• They naturally handle nested structures
• They're perfect for problems involving "most recent" or "last seen"
• The LIFO property solves many problems elegantly

Key Takeaways

• Stacks are LIFO (Last In, First Out)
• Perfect for matching, reversing, and state tracking
• Simple to implement and understand
• Recognize them in problems involving nested structures or "undo" operations

I've found that recognizing when a stack is the right tool makes many problems much easier. The LIFO property is deceptively simple but incredibly useful. I hope this helps you see when stacks can simplify your solutions!