class MyQueue {
     // 从队尾入 从队头出 
    /** Initialize your data structure here. */
    // 一个主栈 一个缓存
    Stack<Integer> stk = new Stack<>();
    Stack<Integer> cache = new Stack<>();
    public MyQueue() {

    }
    // 两个栈实现队列 push 插入队尾 pop 队首元素弹出返回改元素 peek 返回对手 empty 是否为空
    /** Push element x to the back of queue. */
    public void push(int x) {
       stk.push(x);  
    }
    public void copy(Stack<Integer> a, Stack<Integer> b){
        while(a.size()!=0){
            b.push(a.peek());
            a.pop();
        }
    }


    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        copy(stk,cache); //反转一次
        int res = cache.peek(); //此时栈顶就是队头就是栈底
        cache.pop(); //弹出
        copy(cache, stk); //反转回来
        return res;
    }

    /** Get the front element. */
    public int peek() {
        copy(stk, cache); 
        int res = cache.peek(); //得到队头
        copy(cache, stk);
        return res;
    }

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return stk.isEmpty();
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();
 */