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()
*/