LeetCode 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。

• push(x) —— 将元素 x 推入栈中。
• pop() —— 删除栈顶的元素。
• top() —— 获取栈顶元素。
• getMin() —— 检索栈中的最小元素。

示例:

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.

1. 辅助栈

使用一个栈来保存元素，使用另外一个栈来保存最小元素。

/**
 * initialize your data structure here.
 */
var MinStack = function() {
    this.x_stack = [];
    // 因为不加Infinity的话，push 第一个元素的时候，最小栈是空的，还要做空判断，加了Infinity更简单了
    this.min_stack = [Infinity];
};

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

/**
 * @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(x)
 * obj.pop()
 * var param_3 = obj.top()
 * var param_4 = obj.getMin()
 */

2. 使用一个变量 min 来保存最小值

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

/** 
 * @param {number} x
 * @return {void}
 */
MinStack.prototype.push = function(x) {
  if(!this.stack.length) this.min = x;
  this.min = Math.min(x, this.min);
  this.stack.push(x);
};

/**
 * @return {void}
 */
MinStack.prototype.pop = function() {
  this.stack.pop();
  this.min = Math.min(...this.stack);
};

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

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

/**
 * 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()
 */