public class MyStackImpl implements MyStack {
	private Node head;
	private int size;

	public MyStackImpl() {
		head = null;
		size = 0;
	}

	public boolean isEmpty() {
		return size == 0;
	}

	public int pop() {
		if (size == 0) {
			throw new RuntimeException("No element left");
		}
		int result = head.value;
		head = head.next;
		size--;
		return result;
	}

	public void push(int val) {
		if (isEmpty()){
			head = new Node (val);
		} else {
			Node node = new Node (val);
			node.next = head;
			head = node;
		}
		size++;
	}

}