由两个栈组成的队列

编写一个类，用两个栈实现队列，支持队列的基本操作(add,poll,peek)

class TwoSatckQueue {

	private Stack<Integer> StackPush;
	private Stack<Integer> StackPop;

	public TwoSatckQueue() {
		StackPush = new Stack<Integer>();
		StackPop = new Stack<Integer>();
	}

	public void add(int pushInt) {
		StackPush.add(pushInt);
	}

	public int poll() {
		if (StackPush.isEmpty() && StackPop.isEmpty()) {
			throw new RuntimeException("The Stack is empty");
		} else if (StackPop.isEmpty()) {
			while (!StackPush.isEmpty()) {
				StackPop.push(StackPush.pop());
			}
		}
		return StackPop.pop();
	}

	// 返回队列首的元素
	public int peek() {
		if (StackPush.isEmpty() && StackPop.isEmpty()) {
			throw new RuntimeException("The Stack is empty");
		} else if (StackPop.isEmpty()) {
			while (!StackPush.isEmpty()) {
				StackPop.push(StackPush.pop());
			}
		}
		return StackPop.peek();

	}
}