8.栈

                        class Stack{
            constructor(){
                this.items=[];
            }
            push(value){
                this.items.push(value)
            }
            pop(){
                return this.items.pop()
            }
            peek(){
                return this.items[this.items.length-1]
            }
            isEmpty(){
                return this.items.length==0;
            }
            size(){
                return this.items.length;
            }
        }
        var s=new Stack();
        s.push(2);
        s.push(3);
        console.log(s.size())