232. 用栈实现队列

class MyQueue {
    private Stack<Integer> stackPush;
    private Stack<Integer> stackPop;

    public MyQueue() {
        stackPush = new Stack<Integer>();
        stackPop = new Stack<Integer>();
    }

    public void push(int x) {
        stackPush.push(x);
    }

    public int pop() {
        if (stackPush.isEmpty() && stackPop.isEmpty()) {
            throw new RuntimeException("队列为空");
        } else if (stackPop.isEmpty()) {
            while (!stackPush.isEmpty()) {
                stackPop.push(stackPush.pop());
            }
        }

        return stackPop.pop();
    }

    public int peek() {
        if (stackPush.isEmpty() && stackPop.isEmpty()) {
            throw new RuntimeException("队列为空");
        } else if (stackPop.isEmpty()) {
            while (!stackPush.isEmpty()) {
                stackPop.push(stackPush.pop());
            }
        }

        return stackPop.peek();
    }

    public boolean empty() {
        return stackPush.isEmpty() && stackPop.isEmpty();
    }
}