Stacks

What you'll learn

What a stack is, its three operations, and where stacks show up in real software.

Why it matters

The undo button, the browser back button and function calls all use stacks. Many tricky interview problems (matching brackets, evaluating expressions) become easy once you reach for a stack.

Explanation

A stack follows LIFO: Last In, First Out. The newest item is the first one out.

• push(x): put x on top.
• pop(): remove and return the top item.
• peek(): look at the top item without removing it.

All three take O(1) time because you only ever touch the top.

In JavaScript, an array already works as a stack: push adds to the end, pop removes from the end.

Example

Step-by-step walkthrough

Press play to watch the stack grow and shrink.

(empty)

An empty stack. Last in, first out.

Play

1. Three push calls stack A, B, C, with C on top.
2. peek shows the top, C, without removing it.
3. pop() removes and returns C, the last one in.
4. The stack is left holding A, B, and the new top is B.

Try it yourself

Use a stack to reverse the word "stack": push each letter, then pop them all into a result.

Reverse with a stackJavaScript

Editable codeconst word = "stack"; const stack = []; for (const ch of word) { stack.push(ch); } let reversed = ""; while (stack.length > 0) { reversed += stack.pop(); } console.log(reversed);

Run ResetCopyCtrl+Enter to run

Common mistakes

• Popping from an empty stack. Check stack.length first.
• Using shift and unshift (front of the array) and getting O(n) operations.
• Confusing stack and queue. Stack is last in first out. Queue is first in first out.
• Forgetting that peek does not remove.

Quick quiz

Summary

• A stack is last in, first out.
• push, pop, peek are all O(1).
• Use stacks for undo, backtracking and bracket matching.

Next: queues, the opposite of a stack.

const stack = [];

stack.push("A");
stack.push("B");
stack.push("C");
console.log(stack);            // [ 'A', 'B', 'C' ]

console.log(stack.pop());      // C
console.log(stack[stack.length - 1]);  // peek: B
console.log(stack);            // [ 'A', 'B' ]