public class LinkedListBasedStack {
  
  private class Node {
    int value;
    Node next;
  }
  
  Node head = null;
  
  boolean isEmpty () {
    return (head == null);
  }
  
  void push (int value) {
    Node newHead = new Node();
    newHead.next = head;
    newHead.value = value;
    head = newHead;
  }
  
  int pop () {
    int ret = head.value;
    head = head.next;
    return ret;
  }
}