225. 用队列实现栈

一个队列实现

class MyStack {
    Queue<Integer> queue;

    public MyStack() {
        queue = new LinkedList<>();
    }

    public void push(int x) {
        queue.offer(x);
    }

    public int pop() {
        dumpQueue();
        return queue.poll();
    }

    public int top() {
        dumpQueue();
        int ans = queue.peek();
        queue.offer(queue.poll());
        return ans;
    }

    public boolean empty() {
        return queue.isEmpty();
    }

    private void dumpQueue() {
    //把size-1个元素压到队列尾,现在队列头部元素就是栈顶
        int size = queue.size();
        while (--size > 0) {
            queue.offer(queue.poll());
        }
    }
}
/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack obj = new MyStack();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.top();
 * boolean param_4 = obj.empty();
 */

两个队列实现

class MyStack {
    Queue<Integer> mainQueue;// 和栈的出栈顺序保持一致
    Queue<Integer> auxiliaryQueue;// 辅助队列

    public MyStack() {
        mainQueue = new LinkedList<>();
        auxiliaryQueue = new LinkedList<>();
    }

    // 保持出端和栈一样
    public void push(int x) {
        auxiliaryQueue.offer(x);
        while (!mainQueue.isEmpty()) {
            auxiliaryQueue.offer(mainQueue.poll());
        }
        // 交换主队列和辅助队列 可以把名字看作是指针
        Queue<Integer> temp = mainQueue;
        mainQueue = auxiliaryQueue;
        auxiliaryQueue = temp;
    }

    public int pop() {
        return mainQueue.poll(); 
    }

    public int top() {
        return mainQueue.peek();
    }

    public boolean empty() {
        return mainQueue.isEmpty();
    }
}

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

关键

栈和队列的实现和增删查的函数是什么