class StackArr {
    constructor() {
    this.items = [];
    }
    push(el) {
    this.items.push(el)
    }
    pop() {
    this.items.pop()
    }
    top() {
    return this.items[this.items.length - 1]
    }
    isEmpty() {
    return this.items.length == 0
    }
    size() {
    return this.items.length
    }
    toString() {
    return this.items.join('-')
    }
    clear() {
    this.items = []
    }
}

class Stack {
    constructor() {
        this.count = 0
        this.items = {}
    }
    push(el) {
        // 以count为key
        this.items[this.count] = el
        this.count++
    }
    pop() {
        if (!this.count) {
        return undefined
        }
        this.count--
        let res = this.items[this.count]
        delete this.items[this.count]
        return res
    }
    peek() {
        if (!this.count) {
        return undefined
        }
        return this.items[this.count - 1]
    }
    isEmpty() {
        return this.count == 0
    }
    size() {
        return this.count
    }
    toString() {
        if (!this.count) {
        return ''
        }
        // let str = `${this.items[0]}`;
        // for (let i = 1; i < this.count; i++) {
        //   str = `${str},${this.items[i]}`;
        // }
        let str = this.items[0]
        for (let index = 1; index < this.count; index++) {
        str += ',' + this.items[index]
        }
        return str;
    }
    clear() {
        this.items = {}
        this.count = 0
    }
    }