class Stack<T> {
  private items: T[] = [];

  push(item: T): void {
    this.items.push(item);
  }

  pop(): T | undefined {
    return this.items.pop();
  }

  peek(): T | undefined {
    return this.items[this.items.length - 1];
  }

  get size(): number {
    return this.items.length;
  }

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

export function createStack(numbers: number[]): number | undefined {
  const stack = new Stack<number>();
  for (const num of numbers) {
    stack.push(num);
  }
  return stack.pop();
}