//此处填写代码

    var box=document.getElementById('box');

    var btnAdd=document.getElementById('btnAdd');

    var btnRemove=document.getElementById('btnRemove');

    var list=document.getElementById('list');

    var liList=document.getElementsByTagName('li');

    function mouse(){

        for(var i=0;i<3;i++){

            liList[i].onmouseover=function(){

                this.style.background='red';

            }

            liList[i].onmouseout=function(){

                this.style.background='pink';

            }

        }

    }

    mouse();

    btnAdd.onclick=function(){

        var len = list.childElementCount;

        var li=document.createElement('li');

        var txt=document.createTextNode('我是第'+ ++len +'个li');

        li.appendChild(txt);

        list.appendChild(li);

    }

    btnRemove.onclick=function(){

        var len = list.childElementCount;

            list.removeChild(list.children[0]);

            console.log(len);

    }