<style>
.flex:{display:flex}
</style>
<body>
<div>头部</div>
<div class="flex ">
<div class="left ">左侧</div>
<div class="flex middle">
<div>一级</div>
<div class="flex">
<div>二级</div>
<div>三级</div>
</div>
</div>
<div class="fourth ">四级</div>
</div>
</body>
