225. 用队列实现栈

var MyStack = function () {
  this.list = []
}

/**
 * @param {number} x
 * @return {void}
 */
MyStack.prototype.push = function (x) {
  this.list.push(x)
}

/**
 * @return {number}
 */
MyStack.prototype.pop = function () {
  return this.list.pop()
}

/**
 * @return {number}
 */
MyStack.prototype.top = function () {
  return this.list[this.list.length - 1]
}

/**
 * @return {boolean}
 */
MyStack.prototype.empty = function () {
  return this.list.length === 0
}

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