flex布局左右两边固定，中间自适应
<style>
#flex{
display: flex;
background-color: skyblue;
height: 100px;
line-height: 100px;
}
#left,.right{
width: 200px;
}
#center{
background:pink;
/*flex:1*/
/*等价于*/
flex-grow: 1;
text-align: center;
}
</style>
<div id="flex">
<div id="left">我在左边,width: 200px;</div>
<div id="center">我在右边，自适应</div>
<div class="right">我在右边，width：200px</div>
</div>