public class LinkedStack<E> {

    Node top;

    class Node {

        E element;
        Node next;
    }

    public void push(E element) {
        Node newNode = new Node();
        newNode.element = element;
        newNode.next = top;
        top = newNode;
    }

    public E pop() {
        if (top == null) {
            return null;
        }
        E data = top.element;
        top = top.next;
        return data;
    }

    public E peek() {
        if (top == null) {
            return null;
        }
        return top.element;
    }
}