public class Stack<T> {
	private Node<T> first;

	//creates an empty stack
	public Stack() {
		this.first = first;
	}
	//returns true if stack is empty
	public boolean isEmpty() {
		return this.first == null;
	}
	//push value into stack - Last In First Out
	public void push(T x) {
		this.first = new Node<T>(x, this.first);
	}
	//extract the toped value of stacked - Last In First Out
	public T pop() {
		T x = this.first.getValue();
		this.first = this.first.getNext();
		return x;
	}
	//returns the value in the top without extracting him
	public T top() {
		return this.first.getValue();
	}

}