Stack

Stack

A Stack is a Last In, First Out (LIFO) data structure. Think of a stack of plates: you add a plate to the top, and when you take one, you take from the top.

Operations

• push(item) — add an item to the top. O(1)
• pop() — remove and return the top item. O(1)
• peek() — return the top item without removing it. O(1)
• isEmpty() — return true if the stack is empty. O(1)
• size() — return the number of items. O(1)

class Stack {
  constructor() {
    this.items = [];
  }
  push(item) { this.items.push(item); }
  pop() { return this.items.pop(); }
  peek() { return this.items[this.items.length - 1]; }
  isEmpty() { return this.items.length === 0; }
  size() { return this.items.length; }
}

const s = new Stack();
s.push(1);
s.push(2);
s.push(3);
console.log(s.pop());   // 3
console.log(s.peek());  // 2
console.log(s.size());  // 2

Real-World Uses

• Function call stack — when a function calls another, the return address is pushed onto the call stack.
• Undo/redo — text editors push every change onto a stack; undo pops it.
• Expression evaluation — compilers use stacks to evaluate (3 + 4) * 2.
• Backtracking — DFS graph traversal uses a stack (either explicit or via recursion).

Your Task

Implement a Stack class with push, pop, peek, isEmpty, and size methods.