怎么实现鼠标移入第i个li则对应显示第i个div,默认显示第一个LI

html 部分
<ul>
    <li>菜单1</li>
    <li>菜单2</li>
    <li>菜单3</li>
    <li>菜单4</li>
</ul>
<dl>
    <dd>这是内容1</dd>
    <dd>这是内容2</dd>
    <dd>这是内容3</dd>
    <dd>这是内容4</dd>
</dl>

css
* {
    margin: 0;
    padding: 0;
    list-style: none;
}

ul, dl {
    width: 1080px;
    margin: 0 auto;
    overflow: hidden;
}

li:nth-child(1) {
    background: #333;
}

li {
    width: 25%;
    text-align: center;
    line-height: 50px;
    height: 50px;
    box-sizing: border-box;
    border-left: 1px solid #fff;
    border-right: 1px solid #fff;
    background: #000;
    color: #fff;
    float: left;
}

dl {
    position: relative;
}

dd:nth-child(1) {
    display: block;
}

dd {
    width: 100%;
    height: 300px;
    text-align: center;
    line-height: 300px;
    font-size: 50px;
    background: #f2f5f9;
    display: none;
}

js

window.onload = function () {  // html 渲染完成后
    var li = document.getElementsByTagName('li')  // 获取li
    var dd = document.getElementsByTagName('dd')  //获取dd
    for (let i = 0; i < li.length; i++) {     //给每一个li添加鼠标移入事件
        li[i].addEventListener('mouseover', function () {
            //鼠标移入
            setDD(i)
            setLi(i)
        })
    }

    function setLi(num) {
        for (var i = 0; i < li.length; i++) {
            li[i].style.background = "#000"
        }
        li[num].style.background = "#333"
    }

    function setDD(num) {
        for (var i = 0; i < dd.length; i++) {
            dd[i].style.display = "none"
        }
        dd[num].style.display = "block"
    }
}