最小栈问题

问题

定义栈数据结构，请在类型中实现一个能够得到栈的最小元素的min函数。在该栈中，调用min，push及pop的时间复杂度都是O(1)

class MinStack<T> {
	private Stack main = new Stack<T>();
	private Stack child = new Stack<T>();
	Comparator<T> comparator;

	public MinStack(Comparator<T> _comparator) {
		comparator = _comparator;
	}

	public T pop() {
		if (main.isEmpty())
			throw new EmptyStackException();
		else {
			if (!child.isEmpty()
					&& comparator.compare((T) main.peek(), (T) child.peek()) == 0)
				child.pop();
			return (T) main.pop();
		}
	}

	public void push(T t) {
		main.push(t);
		if (child.isEmpty())
			child.push(t);
		else if (comparator.compare(t, (T) child.peek()) < 0)
			child.push(t);
	}

	public T min() {
		if (!child.isEmpty())
			return (T) child.peek();
		else
			throw new EmptyStackException();
	}
}