DSA Basics: Stack

A stack is a linear data structure where the last value added is the first value removed.

This rule is called LIFO, which means Last In, First Out.

Core operations

• push(value) adds a value to the top.
• pop() removes the top value.
• peek() reads the top value without removing it.
• isEmpty() checks whether the stack has no values.

Mental model

Think of a stack as a pile of plates. You add a new plate on top, and you remove the top plate first.

top
  9
  5
  2
bottom

JavaScript implementation

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

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

	pop() {
		return this.items.pop();
	}

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

Where stacks show up

Stacks are useful when the newest unfinished task must be handled first.

• Undo and redo
• Browser history
• Function call stack
• Parentheses validation
• Depth-first search

Complexity

• Push: O(1)
• Pop: O(1)
• Peek: O(1)
• Search by value: O(n)

Practice links

• LeetCode Stack problems
• NeetCode practice roadmap

Use the visualizer below to push and pop from the top.