[LeetCode]225. Implement Stack using Queues

Implement the following operations of a stack using queues.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• empty() -- Return whether the stack is empty.

Notes:

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

• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

思路：使用两个queue来存，push一个数据进来的时候，先存到临时队列中，然后把主队列都复制到临时队列中，然后在临时队列引用给主队列，再把临时队列指向null，这样就保证了每一个新数据进来，都是存在队列的头部

public class MyStack {    private Queue<Integer> queue=new LinkedList<Integer>();    private Queue<Integer> q=new LinkedList<Integer>();    /** Initialize your data structure here. */    public MyStack() {            }        /** Push element x onto stack. */    public void push(int x) {        q.offer(x);        q.addAll(queue);        queue=q;        q.clear();    }        /** 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();    }}/** * 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(); */