栈的最小值

/**
 * initialize your data structure here.
 */
var MinStack = function () {
  this.stack = [];
  this.index = 0;
};

/** 
 * @param {number} x
 * @return {void}
 */
MinStack.prototype.push = function (x) {
  this.stack[this.index++] = x; // 最后一项插入
};

/**
 * @return {void}
 */
MinStack.prototype.pop = function () {
  this.stack.length = this.index - 1; // 通过length减小删除最后一项
  this.index--;
};

/**
 * @return {number}
 */
MinStack.prototype.top = function () { // 返回最后一项
  return this.stack[this.index - 1];
};

/**
 * @return {number}
 */
MinStack.prototype.getMin = function () { // 获取最小值
  return Math.min(...this.stack);
};

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