如何实现左右固定，中间自适应布局

.container
  .left
  .main
  .right

.container {
  display: flex;
}
 
.left {
  flex-basis: 300px;
  flex-shrink: 0;
}
 
.right {
  flex-basis: 300px;
  flex-shrink: 0;
}
 
.main {
  flex-grow: 1;
}

.box {
display: flex
}


.left,.right {
width: 300px
}

.center {
flex: 1
}

 <div class="container">
        <div class="left"></div>
        <div class="main"></div>
        <div class="right"></div>
  </div>

<style>
    /* grid布局 */

    .container {
        height: 20rem;
        margin: 2rem;
        background-color: #eeeeee;
        display: grid;
        grid-template-columns: 300px 1fr 300px;
    }

    .left {
        height: 100%;
        border: 1px solid black;
    }

    .main {
        height: 100%;
    }

    .right {
        height: 100%;
        border: 1px solid;
    }
</style>