class Stack {
  private _data: number[] = [];

  public peek(): number {
    return this._data[this._data.length - 1];
  }

  public push(element: number): void {
    this._data.push(element);
  }

  public pop(): number {
    return this._data.pop();
  }
}

const myStack = new Stack();
myStack.push(1);
myStack.push(2);
myStack.push(3);

console.log(myStack.peek());