초간단 스택

class MyStack <T>{
	ArrayList<T > ar = new ArrayList<>();
	
	public void push(T c) {
		ar.add(c);
	}
	public T pop() {
		T res;
		try {
			res= ar.get(size()-1);
			ar.remove(size()-1);
				
		} catch (Exception e) {
			res = null;
		}
		return res;
		
	}
	public int size() {
		return ar.size();
	}
	public T peek() {
		return ar.get(0);
	}
	
}