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).

使用队列来模仿栈：
使用队列头来模拟栈顶，这样pop和top方法都是直接就取到。
至于push，我们可以在放进新元素的时候把队列中排在这个元素前面的元素一个一个取出来再放回这个队列。

/**
 * @constructor
 */
var Stack = function() {
    this.q1 = [];
};

/**
 * @param {number} x
 * @returns {void}
 */
Stack.prototype.push = function(x) {
    this.q1.push(x);
    var num = this.q1.length-1;
    for (var i = 0; i<num; i++) {
        this.q1.push(this.q1.shift());
    }
};

/**
 * @returns {void}
 */
Stack.prototype.pop = function() {
    return this.q1.shift();
};

/**
 * @returns {number}
 */
Stack.prototype.top = function() {
    return this.q1[0];
};

/**
 * @returns {boolean}
 */
Stack.prototype.empty = function() {
    if (this.q1.length===0)
        return true;
    else 
        return false;
};