leetcode-155 最小栈

var MinStack = function () {
  this.minStack = [];
  // 记录最小值
  this.min = Infinity;
  this.minNumStack = [];
};

/**
 * @param {number} val
 * @return {void}
 */
MinStack.prototype.push = function (val) {
  this.min = Math.min(this.min, val);
  this.minNumStack.push(this.min);
  this.minStack.push(val);
};

/**
 * @return {void}
 */
MinStack.prototype.pop = function () {
  if (this.minStack.length > 0) {
    this.minNumStack.pop();
    if (this.minNumStack.length > 0) {
      this.min = this.minNumStack[this.minNumStack.length - 1];
    } else {
      this.min = Infinity;
    }
    return this.minStack.pop();
  }
};

/**
 * @return {number}
 */
MinStack.prototype.top = function () {
  if (this.minStack.length === 0) return null;
  return this.minStack[this.minStack.length - 1];
};

/**
 * @return {number}
 */
MinStack.prototype.getMin = function () {
  if (this.minStack.length === 0) return null;
  return this.minNumStack[this.minNumStack.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()
 */