Implement LIFO (Last-In-First-Out) stacks and FIFO (First-In-First-Out) queues. Both are fundamental for parsing, undo/redo, and task scheduling.

• LIFO (Last-In-First-Out) stacks
• FIFO (First-In-First-Out) queues

class Stack {
  constructor() { this.items = []; }
  push(x) { this.items.push(x); }
  pop() { return this.items.pop(); }
  peek() { return this.items[this.items.length - 1]; }
}
// ...

• Stack LIFO
• Queue FIFO

• Stack: push/pop from end; Queue: enqueue at end, dequeue from front
• For efficiency, queues should use linked list or circular array, not array.shift()
• Common use: parenthesis matching (stack), BFS (queue)

Implement LIFO (Last-In-First-Out) stacks and FIFO (First-In-First-Out) queues. Both are fundamental for parsing, undo/redo, and task scheduling.

• LIFO (Last-In-First-Out) stacks
• FIFO (First-In-First-Out) queues

class Stack {
  constructor() { this.items = []; }
  push(x) { this.items.push(x); }
  pop() { return this.items.pop(); }
  peek() { return this.items[this.items.length - 1]; }
}
// ...

• Stack LIFO
• Queue FIFO

• Stack: push/pop from end; Queue: enqueue at end, dequeue from front
• For efficiency, queues should use linked list or circular array, not array.shift()
• Common use: parenthesis matching (stack), BFS (queue)