const MinStack = function () {
  this.data = []; // 内部使用数组维护栈元素
  this.minstack = [Infinity]; // 利用栈结构维护最小元素
};

/**
 * @param {number} val
 * @return {void}
 */
MinStack.prototype.push = function (val) {
  // 判断当前插入的元素是否比栈的最小元素还小
  if (val < this.minstack[this.minstack.length - 1]) this.minstack.push(val);
  this.data.push(val);
};

/**
 * @return {void}
 */
MinStack.prototype.pop = function () {
  // 判断栈顶的元素是否是最小元素，如果是，最小元素栈顶的元素也要删除
  if (this.data[this.data.length - 1] === this.minstack[this.minstack.length - 1]) {
    this.minstack.pop();
  }
  if (this.data.length) this.data.pop();
};

/**
 * @return {number}
 */
MinStack.prototype.top = function () {
  if (this.data.length) return this.data[this.data.length - 1];
};

/**
 * @return {number}
 */
MinStack.prototype.getMin = function () {
  return this.minstack[this.minstack.length - 1];
};

/**
 * Your MinStack object will be instantiated and called as such:
 * var obj = new MinStack()
 * obj.push(val)
 * obj.pop()
 * var param_3 = obj.top()
 * var param_4 = obj.getMin()
 */