No.155 最小栈

设计一个支持 push ，pop ，top 操作，并能在常数时间内检索到最小元素的栈。

实现 MinStack 类:

• MinStack() 初始化堆栈对象。
• void push(int val) 将元素 val 推入堆栈。
• void pop() 删除堆栈顶部的元素。
• int top() 获取堆栈顶部的元素。
• int getMin() 获取堆栈中的最小元素。

示例 1:

输入：
["MinStack","push","push","push","getMin","pop","top","getMin"]
[[],[-2],[0],[-3],[],[],[],[]]

输出：
[null,null,null,null,-3,null,0,-2]

解释：
MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin(); --> 返回 -3.
minStack.pop();
minStack.top(); --> 返回 0.
minStack.getMin(); --> 返回 -2.

提示：

• -2^31 <= val <= 2^31 - 1
• pop、top 和 getMin 操作总是在 非空栈 上调用
• push, pop, top, and getMin 最多被调用 3 * 10^4 次

解题思路

实现

构造辅助栈

var MinStack = function () {
  this.x_stack = [];
  this.min_stack = [Number.MAX_VALUE];
};

/**
 * @param {number} val
 * @return {void}
 */
MinStack.prototype.push = function (val) {
  this.x_stack.push(val);
  this.min_stack.push(Math.min(val, this.min_stack[this.min_stack.length - 1]));
};

/**
 * @return {void}
 */
MinStack.prototype.pop = function () {
  this.x_stack.pop();
  this.min_stack.pop();
};

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

/**
 * @return {number}
 */
MinStack.prototype.getMin = function () {
  return this.min_stack[this.min_stack.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()
 */

链表实现

function Node(val, min, next) {
  this.val = val || 0;
  this.min = min || 0;
  this.next = next || null;
}

var MinStack = function () {
  this.head = null;
};

/**
 * @param {number} val
 * @return {void}
 */
MinStack.prototype.push = function (val) {
  if (!this.head) {
    this.head = new Node(val, val, null);
  } else {
    this.head = new Node(val, Math.min(val, this.head.min), this.head);
  }
};

/**
 * @return {void}
 */
MinStack.prototype.pop = function () {
  this.head = this.head.next;
};

/**
 * @return {number}
 */
MinStack.prototype.top = function () {
  return this.head.val;
};

/**
 * @return {number}
 */
MinStack.prototype.getMin = function () {
  return this.head.min;
};
/**
 * 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()
 */