155. 最小栈（算法200题-30）

class Node {
    constructor(val, min, next = null) {
        this.val = val
        this.min = min
        this.next = next
    }
}
class MinStack {
    constructor() {
        this.head = null
        this.min = null
    }
    push(val) {
        if(!this.head) {
            this.head = new Node(val, val)
            this.min = val
        } else {
            const temp = this.head
            this.min = Math.min(val, this.min)
            this.head = new Node(val, this.min)
            this.head.next = temp
        }
    }
    pop() {
        if(!this.head) return
        const popVal = this.head.val
        this.head = this.head.next
        this.min = this.head && this.head.min
        return popVal
    }
    getMin() {
        return this.min
    }
    top() {
        return this.head.val
    }
}