Dual

225. Implement Stack using Queues

Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty).

Implement the MyStack class:

• void push(int x) Pushes element x to the top of the stack.
• int pop() Removes the element on the top of the stack and returns it.
• int top() Returns the element on the top of the stack.
• boolean empty() Returns true if the stack is empty, false otherwise.

Notes:

• You must use only standard operations of a queue, which means that only push to back, peek/pop from front, size and is empty operations are valid.
• Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue) as long as you use only a queue's standard operations.

双队列写法

class MyStack {
    Queue<Integer> outQueue;
    Queue<Integer> inQueue;

    /** Initialize your data structure here. */
    public MyStack() {
        outQueue = new ArrayDeque<>();
        inQueue = new ArrayDeque<>();
    }

    /** Push element x onto stack. */
    public void push(int x) {
        inQueue.offer(x);
        while (!outQueue.isEmpty()) {
            inQueue.offer(outQueue.poll());
        }
        Queue<Integer> temp = outQueue;
        outQueue = inQueue;
        inQueue = temp;
    }

    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return outQueue.poll();
    }

    /** Get the top element. */
    public int top() {
        return outQueue.peek();
    }

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

单队列写法：

class MyStack {
    Queue<Integer> queue;

    /** Initialize your data structure here. */
    public MyStack() {
        queue = new ArrayDeque<>();
    }

    /** Push element x onto stack. */
    public void push(int x) {
        queue.offer(x);
        for (int i = 1; i < queue.size(); i++) {
            queue.offer(queue.poll());
        }
    }

    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return queue.poll();
    }

    /** Get the top element. */
    public int top() {
        return queue.peek();
    }

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