class ConcurrentStack<E> {
	
	private final AtomicReference<Entry<E>> pointer = new AtomicReference<>();
	
	public void push(E element) {
		boolean finish = false;
		final Entry<E> newTop = new Entry<>(element);
		do {
			Entry<E> currentTop = pointer.get();
			newTop.setPrev(currentTop);
			finish = pointer.compareAndSet(currentTop, newTop);
		} while(!finish);
	}
	
	public E pop() {
		E ret = null;
		boolean finish = false;
		do {
			Entry<E> currentTop = pointer.get();
			Entry<E> newTop = null;
			if(currentTop != null) {
				ret = currentTop.element;	
				newTop = currentTop.prev;
			} else {
				return null;
			}
			finish = pointer.compareAndSet(currentTop, newTop);
		} while(!finish);
		return ret;
	}
	
	public long size() {
		Entry<E> current = pointer.get();
		return current != null?current.index:0;
	}
	
	private static class Entry<E> {
		private E element; 
		private Entry<E> prev; 
		private long index = 1;

		Entry(E element) {
			this.element = element;
		}
		
		void setPrev(Entry<E> prev) {
			this.prev = prev;
			if(prev != null) {
				index = prev.index + 1;
			}
		}
	}
}

if(concurrentStack.size() > 100) {
  //increase number of popping threads
}