css如何让自适应宽度的块居中显示

  <div class=‘footer‘>      
      <ul>
          <li>网站首页</li>
          <li>企业招聘</li>
          <li>联系我们</li>
          <li>讲师招聘</li>
      </ul>
  </div>

    .footer{
        width: 100%;
        height: 100px;
        background: black;
        position: fixed;
        bottom: 0;
        text-align: center;
    }
    .footer ul{
        display: inline-block;
        color: white;
    }
    .footer li{
        height: 100px;
        line-height: 100px;
        float: left;
        margin-right: 20px
    }