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).

一刷
题解：
one queue的思路：
push之后，把queue的在push之前的元素都一一poll, 再add到末尾。

public class MyStack {
    
    Queue<Integer> queue;

    /** Initialize your data structure here. */
    public MyStack() {
        queue = new LinkedList<Integer>();
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        queue.add(x);
        for(int i=0; i<queue.size()-1; i++){
            queue.add(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();
    }
}

/**
 * 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();
 */

方法2，两个queue, 并没有显著的差别
push时，若q1是empty, 则q1.add(a), 并且把q2中的元素全部poll()到q1中

public class MyStack {
    private Queue<Integer> q1;
    private Queue<Integer> q2;

    /** Initialize your data structure here. */
    public MyStack() {
        q1 = new LinkedList<>();
        q2 = new LinkedList<>();
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        if(q1.isEmpty()){
            q1.add(x);
            for(int i=0; i<q2.size(); i++){
                q1.add(q2.poll());
            }
        }
        else{
            q2.add(x);
            for(int i=0; i<q1.size(); i++){
                q2.add(q1.poll());
            }
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        if(q1.isEmpty()) return q2.poll();
        else return q1.poll();
    }
    
    /** Get the top element. */
    public int top() {
        if(q1.isEmpty()) return q2.peek();
        else return q1.peek();
    }
    
    /** Returns whether the stack is empty. */
    public boolean empty() {
        return q1.isEmpty() && q2.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();
 */